See then the aptness of the figure. The figure of the serpent has the appearance of the beast, but not its poison: in the same way Christ came in the likeness of sinful flesh, being free from sin. By Christ’s being lifted up, understand His being suspended on high, by which suspension He sanctified the air, even as He had sanctified the earth by walking upon it. Herein too is typified the glory of Christ: for the height of the cross was made His glory for in that He submitted to be judged, He judged the prince of this world; for Adam died justly, because he sinned; our Lord unjustly, because He did no sin. So He overcame him, who delivered Him over to death, and thus delivered Adam from death. And in this the devil found himself vanquished, that he could not upon the cross torment our Lord into hating His murderers: but only made Him love and pray for them the more. In this way the cross of Christ was made His lifting up, and glory. (Theophylus of Antioch quoted by Saint Thomas Aquinas. Catena Aurea on Jn 3:14–15)

There a serpent bit and a serpent healed, here death destroyed and a Death saved. But the snake which destroyed had venom, that which saved was free from venom; and so again was it here, for the death which slew us had sin with it, as the serpent had venom; but the Lord’s Death was free from all sin, as the brazen serpent from venom. For, says Peter, ‘He did no sin, neither was guile found in His mouth.’ (1 Pet 2:22). And this is what Paul also declares, ‘And having spoiled principalities and powers, He made a show of them openly, triumphing over them in it’ (Col 2:16). For as some noble champion by lifting on high and dashing down his antagonist, renders his victory more glorious, so Christ, in the sight of all the world, cast down the adverse powers, and having healed those who were smitten in the wilderness, delivered them from all venomous beasts that vexed them, by being hung upon the Cross. Yet He did not say, ‘must hang,’ but, ‘must be lifted up’ (Acts 28:4); for He used this which seemed the milder term, on account of His hearer, and because it was proper to the type. (Saint John Chrysostom. Homilies on the Gospel of Saint John, Homily 27)

What are the biting serpents? Sins, from the mortality of the flesh. What is the serpent lifted up? The Lord’s death on the cross. For as death came by the serpent, it was figured by the image of a serpent. The serpent’s bite was deadly, the Lord’s death is life-giving. A serpent is gazed on that the serpent may have no power. What is this? A death is gazed on, that death may have no power. […] Meanwhile brethren, that we may be healed from sin, let us now gaze on Christ crucified; for ‘as Moses,’ says He, ‘lifted up the serpent in the wilderness, so must the Son of man be lifted up; that whosoever believes in Him may not perish, but have everlasting life.’ Just as they who looked on that serpent perished not by the serpent’s bites, so they who look in faith on Christ’s death are healed from the bites of sins. But those were healed from death to temporal life; while here He says, ‘that they may have everlasting life.’ Now there is this difference between the figurative image and the real thing: the figure procured temporal life; the reality, of which that was the figure, procures eternal life. (Saint Augustine of Hippo. Tractates on the Gospel of Saint John, XII, 11)

This bronze serpent has become a figure of Christ ‘raised upon’ the cross. The exegetes see therein the symbolic announcement of the fact that man, who contemplates the cross with faith, ‘receives life’…and life signifies victory over sin, and the state of grace in the human soul. (John Paul II. Homily to University students preparing for Easter, no. 3, March 30, 1982)

This Word of God made flesh and dwelt amongst us. […] This was the way in which, though immortal, he was able to die; the way in which he chose to give life to mortal men: he would first share with us, and then enable us to share with him. Of ourselves we had no power to live, nor did he of himself have the power to die. In other words, he performed the most wonderful exchange with us. Through us, he died; through him, we shall live. The death of the Lord our God should not be a cause of shame for us; rather, it should be our greatest hope, our greatest glory. In taking upon himself the death that he found in us, he has most faithfully promised to give us life in him, such as we cannot have of ourselves. He loved us so much that, sinless himself, he suffered for us sinners the punishment we deserved for our sins. How then can he fail to give us the reward we deserve for our righteousness, for he is the source of righteousness? How can he, whose promises are true, fail to reward the saints when he bore the punishment of sinners, though without sin himself? Brethren, let us then fearlessly acknowledge, and even openly proclaim, that Christ was crucified for us; let us confess it, not in fear but in joy, not in shame but in glory. (Saint Augustine of Hippo. Sermon Guelf 3 from the Office of Readings, Monday of Holy Week)

He [the Word] becomes an infant […]. He reveals as much of Himself as He knows the recipient can accept; He does not diminish the manifestation of His own greatness out of lack of generosity but estimates the receptive capacity of those who desire to see Him. In this way the divine Logos is eternally made manifest m different modes of participation, and yet remains eternally invisible to all in virtue of the surpassing nature of His hidden activity. That is why the apostle, when wisely considering the power of this hidden activity, says, ‘Jesus Christ the same yesterday, and today, and throughout the ages’ (Heb 13:8); for he sees the hidden activity as something which is always new and never becomes outmoded through being embraced by the intellect. […] Christ our God is born and becomes man by adding to Himself flesh endowed with an intellective soul. He who from non-being brings created things into being is Himself born supranaturally of a Virgin who does not thereby lose her virginity. For just as He Himself became man without changing His nature or altering His power, so He makes her who bore Him a Mother while keeping her a Virgin. In this way He reveals one miracle through another miracle, at the same time concealing the one with the other. This is because in Himself, according to His essence. God always remains a mystery. He expresses His natural hiddenness in such a way that He makes it the more hidden through the revelation. Similarly, in the case of the Virgin who bore Him, He made her a Mother in such a way that by conceiving Him the bonds of her virginity became even more in- dissoluble. […] As man I deliberately transgressed the divine commandment, when the devil, enticing me with the hope of divinity (cf. Gen. 3:5), dragged me down from my natural stability into the realm of sensual pleasure; and he was proud to have thus brought death into existence, for he delights in the corruption of human nature. Because of this, God became perfect man, taking on everything that belongs to human nature except sin (cf. Heb. 4:15); and indeed sin is not part of human nature. In this way, by enticing the insatiable serpent with the bait of the flesh, He provoked him to open his mouth and swallow it. This flesh proved poison to him, destroying him utterly by the power of the Divinity within it; but to human nature it proved a remedy restoring it to its original grace by that same power of the Divinity within it. For just as the devil poured out his venom of sin on the tree of knowledge and corrupted human nature once it had tasted it, so when he wished to devour the flesh of the Master he was himself destroyed by the power of the Divinity within it. […] How did He who is wholly God by nature become wholly man by nature, not renouncing either nature in any way at all, neither the divine, through which He is God, nor ours, through which He became man? Faith alone can embrace these mysteries, for it is faith that makes real for us things beyond intellect and reason (cf. Heb. 11:1). Because Adam disobeyed, human nature has come to be generated through sensual pleasure; banishing such pleasure from human nature, the Lord had nothing to do with engendering by means of seed. Because the woman transgressed the commandment, the generation of human nature begins in pain (cf. Gen. 3:16); expelling this from human nature through His birth, the Lord did not allow her who bore Him to lose her virginity. He did this in order to expel from human nature both pleasure deliberately sought and the resulting unsought pain, becoming the destroyer of those things which He did not create. Through this He also mystically taught us to embark of our own accord on another way of life, one perhaps begun in pain and labor but nevertheless ending in divine pleasure and everlasting gladness. That is why He who made man became a man and was born as a man, so that He might save man and, by healing our passions through His passion, might Himself supra-naturally destroy the passions that were destroying us, in His compassion renewing us in the spirit through His privations in the flesh. (Saint Maximus the Confessor. In: The Philokalia: The Complete Text, vol. 2, pg.166–168)

In this way, then, Jesus has been made truly similar to men, assuming the condition of a slave, as the Letter to the Philippines proclaims (cf. 2:7). But the Epistle to the Hebrews, in speaking of Him as ‘high priest of the good things that have to come’ (9: 11), confirms and clarifies that ‘we do not have a high priest who is unable to sympathize with our weaknesses, but one who has similarly been tested in every way, yet without sin’ (4:15). Truly ‘he did not know sin’ although St. Paul would say that God ‘made him to be sin who did not know sin, so that we might become the righteousness of God in him’ (2Cor 5:21). Jesus himself could present the challenge: ‘Can any of you charge me with sin?’ (Jn 8:46). And this is the faith of the Church: ‘Sine peccato conceptus, natus et mortuus’. The Council of Florence (Decree pro Iacob) proclaims so, in harmony with all the Tradition: Jesus “who without sin was conceived, born, and died.” He is the truly just and holy man. We repeat with the New Tesament, with the Creed, and with the Council: “Jesus Christ has truly been made one of us, like us in all things except sin (cf. Heb 4:15).” (John Paul II. General audience, no. 9, February 3, 1988)

Christ, as the man who suffers really and in a terrible way in the Garden of Olives and on Calvary, addresses Himself to the Father- that Father whose love He has preached to people, to whose mercy He has borne witness through all of His activity. But He is not spared – not even He – the terrible suffering of death on the cross: ‘For our sake God made him to be sin who knew no sin, (2Cor 5:21) St. Paul will write, summing up in a few words the whole depth of the cross and at the same time the divine dimension of the reality of the Redemption. Indeed this Redemption is the ultimate and definitive revelation of the holiness of God, who is the absolute fullness of perfection: fullness of justice and of love, since justice is based on love, flows from it and tends towards it. In the passion and death of Christ – in the fact that the Father did not spare His own Son, but ‘for our sake made him sin’ (2Cor 5:21) – absolute justice is expressed, for Christ undergoes the passion and cross because of the sins of humanity. This constitutes even a ‘superabundance’ of justice, for the sins of man are ‘compensated for’ by the sacrifice of the Man-God. (John Paul II. Encyclical Dives in misericordia, no. 7, November 30, 1980)

St Paul has summed it for us in the Second Letter to the Corinthians, today’s Second Reading: ‘For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God’ (2 Cor 5:11). Our possibility of receiving divine forgiveness depends essentially on the fact that God himself, in the person of his Son, wished to share in our human condition, but not in the corruption of sin. (Benedict XVI. Homily on Ash Wednesday, February 12, 2012)

In one way because it was the custom of the Old Law to call a sacrifice for sin “sin”: ‘They feed on the sin of my people’ (Hos. 4:8), i.e., the offerings for sin. Then the sense is: he made him to be sin, i.e., the victim of sacrifice for sin. (Saint Thomas Aquinas. Commentary on the Second epistle to the Corinthian, no. 201)

In another way, because sin is sometimes taken for the likeness of sin, or the punishment of sin: ‘God sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh’ (Rom. 8:3). Then the sense is: he made him to be sin, i.e., made him assume mortal and suffering flesh. (Saint Thomas Aquinas. Commentary on the Second epistle to the Corinthian, no. 201)

In a third way, because one thing is said to be this or that, not because it is so, but because man considers it such. Then the sense is: he made him to be sin, i.e., made him regarded a sinner: ‘He was numbered with the transgressors’ (Is. 53:12). (Saint Thomas Aquinas. Commentary on the Second epistle to the Corinthian, no. 201)

‘The Word became flesh and dwelt among us’ (Jn 1:14), that is, in the flesh that he took from a human being and which he animated with the spirit of rational life. The uniqueness of each nature being preserved and combined in one person […] an inviolable nature was joined to a passible nature […] Therefore, the true God was born in the complete and perfect nature of true man, complete in his nature and complete in ours […] He assumed the form of a servant without the defilement of sin, enriching the human without diminishing the divine, because that self-emptying, through which the invisible rendered himself visible…, was an inclination of mercy not a defect of power. The Son of God, therefore, descending from his heavenly throne, enters into the infirmities of this world; and, not leaving the Father’s glory, he is generated in a new order and a new birth. (Denzinger-Hünermann 292–294. Saint Leo I, the Great, Dogmatic epistle Lectis dilectionis tua, to Bishop Flavian of Constantinople, June 13, 449)

Hence, we confess one will of our Lord Jesus Christ also, because surely our nature, not our guilt was assumed by the Godhead, that certainly, which was created before sin, not that which was vitiated after the transgression. For Christ…was conceived of the Holy Spirit without sin, and was also born of the holy and immaculate Virgin mother of God without sin, experiencing no contagion of our vitiated nature. (Denzinger-Hünermann 487. Honorius I, Epistle Scripta fraternitatis vestrae to Sergius, Patriarch of Constantinople in the year 634)

In this form of assumed human nature we believe according to the truth of the Gospels that He was conceived without sin, born without sin, and died without sin, who alone for us became sin (2Cor 5:21), that is, a sacrifice for our sin. And yet He endured His passion without detriment to His divinity, for our sins, and condemned to death and to the cross, He accepted the true death of the body; also on the third day, restored by His own power, He arose from the grave. (Denzinger-Hünermann 539. Council of Toledo XI, Creed of Faith, November 7, 675)

What we do ask is that He may deliver us from sins. This is the interpretation of St. Luke, who, instead of debts, makes use of the word sins, because by their commission we become guilty before God and incur a debt of punishment, which we must pay either by satisfaction or by suffering. It was of this debt that Christ the Lord spoke by the mouth of His Prophet: Then did I pay that which I took not away. (Catechism of Trent, 4500)

For we do not have a high priest who is unable to sympathize with our weaknesses, but one who has similarly been tested in every way, yet without sin. (Heb 4:15)

For our sake he made him to be sin who did not know sin, so that we might become the righteousness of God in him. (2 Cor 5:21)

For to this you have been called, because Christ also suffered for you, leaving you an example that you should follow in his footsteps. ‘He committed no sin, and no deceit was found in his mouth.’ When he was insulted, he returned no insult; when he suffered, he did not threaten; instead, he handed himself over to the one who judges justly. He himself bore our sins in his body upon the cross, so that, free from sin, we might live for righteousness. (1Pet 2:21–24)

One and He alone is without sin, the mediator of God and of men, the man Christ Jesus (cf. 1Tim 2,5) who was conceived and born free among the dead (Ps 87:6). Thus in the dispensation of His sacred flesh, He never had two contrary wills, nor did the will of His flesh resist the will of His mind. . . Therefore, knowing that there was no sin at all in Him when He was born and lived, we fittingly say and truthfully confess one will in the humanity of His sacred dispensation; and we do not preach two contrary wills, of mind and of flesh, as in a pure man, in the manner certain heretics are known to rave. In accord with this method, then, our predecessor (already mentioned) [Honorius] is known to have written to the (aforenamed) Sergius the Patriarch who was asking questions, that in our Savior two contrary wills did not exist internally, that is, in His members, since He derived no blemish from the transgression of the first man. (Denzinger-Hünermann 496–497. John IV, Epistle Dominus qui dixit to Constantius Emporor, 641)

We confess also that each one of his natures has its own natural properties: the divine has everything belonging to the divine nature, and the human has all that belongs to human nature, without any sin. We acknowledge that both (natures) belong without any confusion, inseparably and immutable, to one and the same incarnate God, the Word become man; only intelligence distinguishes what is united in order to avoid the error of confusion. Equally we detest the blasphemy of separation and of confusion. (Denzinger-Hünermann 543. Agatho. Letter Consideranti mihi to the Emperors, March 27, 680)

Christ assumed human defects in order to satisfy for the sin of human nature, and for this it was necessary for Him to have the fullness of knowledge and grace in His soul. Hence Christ ought to have assumed those defects which flow from the common sin of the whole nature, yet are not incompatible with the perfection of knowledge and grace. And thus it was not fitting for Him to assume all human defects or infirmities. For there are some defects that are incompatible with the perfection of knowledge and grace, as ignorance, a proneness towards evil, and a difficulty in well-doing. (Saint Thomas Aquinas. Summa Theologica III, q. 14, a. 4, sol)

By sending his own Son in the form of a slave, in the form of a fallen humanity, on account of sin, God ‘made him to be sin who knew no sin, so that in him we might become the righteousness of God.’ (cf. Phil 2:7) Jesus did not experience reprobation as if he himself had sinned. (cf. Jn 8:46) But in the redeeming love that always united him to the Father, he assumed us in the state of our waywardness of sin, to the point that he could say in our name from the cross: ‘My God, my God, why have you forsaken me?’ (Mk 15:34; Ps 22:2; cf. Jn 8:29) Having thus established him in solidarity with us sinners, God ‘did not spare his own Son but gave him up for us all’, so that we might be ‘reconciled to God by the death of his Son’ (Rom 8:32; 5:10). (Catechism of the Catholic Church, no. 602–603)

Christ manifested man fully to man exactly because of the fact that He ‘had not known sin’. For sin is by no means an enrichment to man. Entirely to the contrary: it depreciates him, diminishes him, deprives him of his rightful plenitude (cf. Gaudium et Spes, 13). The recuperation, the salvation of fallen man is the fundamental response to the question regarding the reason of the Incarnation. (John Paul II. General Audience, no. 10, February 3, 1988)

Him who knew no sin. Experimentally, says St. Thomas, Christ knew no sin, though by simple knowledge He did, for He did no sin. Hath made Him to be sin for us. For us, says Illyricus, who were sin; because, he says, sin is the substance and form of our soul. But to say this of ourselves is folly, of Christ blasphemy. (Cornelius a Lápide. Commentary on the Second letter of Saint Paul to the Corinthians, Ver. 21)

Can any of you charge me with sin? If I am telling the truth, why do you not believe me? Whoever belongs to God hears the words of God; for this reason you do not listen, because you do not belong to God. (Jn 8:46–47)

This is the interpretation of St. Luke, who, instead of debts, makes use of the word sins, because by their commission we become guilty before God and incur a debt of punishment, which we must pay either by satisfaction or by suffering. It was of this debt that Christ the Lord spoke by the mouth of His Prophet: ‘Then did I pay that which I took not away’(Ps 68: 5). From these words of God we may understand that we are not only debtors, but also unequal to the payment of our debt, the sinner being of himself utterly incapable of making satisfaction. (Catechism of Trent, 4500)

Let us set before the eyes of our soul a man gravely wounded, almost to the point of breathing his last breath, lying naked upon the dust of the earth. In his desire that a doctor arrive, he groans and asks he who understands his state to have compassion. Well, sin is a wound of the soul. You who are wounded, realize that your doctor is before you and uncover to him the wounds of your sins. (Gregory I, the Great. Exposition on the Seven penitential psalms: PL 79, 581s)

Sin is not just a psychological and social matter, but an event that corrodes the relationship with God, violating his law, refusing his plan in history and overturning his set of values, ‘putting darkness for light and light for darkness’, in other words, ‘calling evil good and good evil’ (cf. Is 5,20). Before finally injuring man, sin is first and foremost a betrayal of God. (John Paul II. General audience, no. 3, May 8, 2002)

The death on the cross, painful and excruciating, was also “sacrifice of expiation”, that makes us understand both the gravity of sin, which is rebellion against God and rejection of his love, and also the marvelous redeeming work of Christ, which in expiating for humanity has restored grace to us, that is, the participation in very the Trinitarian life of God and the inheritance of his eternal happiness. (John Paul II. General audience, no. 3, March 22, 1989)

For the wicked are at war with God, who is offended beyond belief at their crimes; hence the Apostle says: Wrath and indignation, tribulation and anguish upon every soul of man that worketh evil. Although the sinful act is transient, yet the sin by its guilt and stain remains; and the imminent wrath of God pursues it, as the shadow does the body. (Catechism of Trent, 4500)

Was it necessary for Jesus Christ to suffer as much as He actually did?
No, it was not absolutely necessary for Jesus Christ to suffer as much as He did, because each of His acts being of infinite value, the least of His sufferings would have sufficed for our redemption.
Why, then, did Jesus suffer so much?
Jesus Christ suffered so much in order to satisfy divine justice all the more abundantly; to display His love for us still more; and to inspire us with the deepest horror of sin. (Catechism of Saint Pius X. The fourth article of the Creed, no. 12–13)

Realizing that you were ransomed from your futile conduct, handed on by your ancestors, not with perishable things like silver or gold but with the precious blood of Christ as of a spotless unblemished lamb. (1 Pet 1:18–19)