Now there was a man of the Pharisees, named Nicodemus, a ruler of the Jews. This man came to Jesus by night and said to him, “Rabbi, we know that you are a teacher come from God; for no one can do these signs that you do, unless God is with him.” Jesus answered him, “Truly, truly, I say to you, unless one is born anew, he cannot see the kingdom of God.” Nicodemus said to him, “How can a man be born when he is old? Can he enter a second time into his mother’s womb and be born?” Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God.” (Jn 3:1-5)

But though He died for all, yet all do not receive the benefit of His death, but those only to whom the merit of His passion is communicated […] In which words is given a brief description of the justification of the sinner, as being a translation from that state in which man is born a child of the first Adam, to the state of grace and of the adoption of the sons of God through the second Adam, Jesus Christ, our Savior. This translation however cannot, since promulgation of the Gospel, be effected except through the laver of regeneration or its desire, as it is written: Unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God. (Council of Trent. Chapter III, Session VI. Who are justified through Christ. A Brief Description of the Justification of the Sinner and its mode in the State of Grace. Celebrated on the thirteenth day of January, 1547 under Pope Paul III)

Men are bound to that without which they cannot obtain salvation. Now it is manifest that no one can obtain salvation but through Christ; wherefore the Apostle says (Rm 5, 18): “As by the offense of one unto all men unto condemnation; so also by the justice of one, unto all men unto justification of life.” But for this end is Baptism conferred on a man, that being regenerated there by, he may be incorporated in Christ, by becoming His member: wherefore it is written (Ga 3, 27): “As many of you as have been baptized in Christ, have put on Christ.” Consequently it is manifest that all are bound to be baptized: and that without Baptism there is no salvation for men. (Saint Thomas Aquinas. Summa Theologica III, q. 68, a. 1)

 With Baptism we become children of God in his only-begotten Son, Jesus Christ. Rising from the waters of the Baptismal font, every Christian hears again the voice that was once heard on the banks of the Jordan River:  “You are my beloved Son; with you I am well pleased” (Lc 3, 22). From this comes the understanding that one has been brought into association with the beloved Son, becoming a child of adoption (cf. Ga 4, 4-7) and a brother or sister of Christ. In this way the eternal plan of the Father for each person is realized in history:  “For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the first-born among many brethren.” (Rm 8, 29) (John Paul II. Post-Synodal Apostolic Exhortation Christifideles laici, n.11)

Regenerated as “Children in the Son”, the baptized are inseparably joined together as “members of Christ and members of the body of the Church” as the Council of Florence teaches. Baptism symbolizes and brings about a mystical but real incorporation into the crucified and glorious body of Christ. […] In the words of Saint Paul we find again the faithful echo of the teaching of Jesus himself, which reveals the mystical unity of Christ with his disciples and the disciples with each other, presenting it as an image and extension of that mystical communion that binds the Father to the Son and the Son to the Father in the bond of love, the Holy Spirit (cf. Jn 17, 21).  (John Paul II. Post-Synodal Apostolic Exhortation Christifideles laici, n.12)

This Sacred Council wishes to turn its attention firstly to the Catholic faithful. Basing itself upon Sacred Scripture and Tradition, it teaches that the Church, now sojourning on earth as an exile, is necessary for salvation. Christ, present to us in His Body, which is the Church, is the one Mediator and the unique way of salvation. In explicit terms He Himself affirmed the necessity of faith and baptism and thereby affirmed also the necessity of the Church, for through baptism as through a door men enter the Church. Whosoever, therefore, knowing that the Catholic Church was made necessary by Christ, would refuse to enter or to remain in it, could not be saved. (Vatican Council II. Dogmatic Constitution of the Church: Lumen gentium, n.14)

Through Baptism the Christian is sacramentally assimilated to Jesus, who in his own baptism anticipates his death and resurrection. The Christian must enter into this mystery of humble self-abasement and repentance, go down into the water with Jesus in order to rise with him, be reborn of water and the Spirit so as to become the Father’s beloved son in the Son and “walk in newness of life” (Rm 6:4). (Catechism of the Catholic Church, 537)

The People of God is marked by characteristics that clearly distinguish it from all other religious, ethnic, political, or cultural groups found in history: It is the People of God: God is not the property of any one people. But he acquired a people for himself from those who previously were not a people:  “a chosen race, a royal priesthood, a holy nation.” (1 Pet 2:9) One becomes a member of this people not by a physical birth, but by being “born anew”, “a birth of water and the Spirit” (Jn 3:3-5) that is, by faith in Christ, and Baptism.  (Catechism of the Catholic Church, no. 782)

This shameful font of indifferentism gives rise to that absurd and erroneous proposition which claims that liberty of conscience must be maintained for everyone. It spreads ruin in sacred and civil affairs, though some repeat over and over again with the greatest impudence that some advantage accrues to religion from it.  “But the death of the soul is worse than freedom of error”, as Augustine was wont to say. When all restraints are removed by which men are kept on the narrow path of truth, their nature, which is already inclined to evil, propels them to ruin. Then truly  “the bottomless pit” is open from which John saw smoke ascending which obscured the sun, and out of which locusts flew forth to devastate the earth. Thence comes transformation of minds, corruption of youths, contempt of sacred things and holy laws – in other words, a pestilence more deadly to the state than any other. Experience shows, even from earliest times, that cities renowned for wealth, dominion, and glory perished as a result of this single evil, namely immoderate freedom of opinion, license of free speech, and desire for novelty. (Gregory XVI. Encyclical Mirari vos, no. 14,  August 15, 1832)

Some men, indeed do not attack the truth willfully, but work in heedless disregard of it. They act as though God had given us intellects for some purpose other than the pursuit and attainment of truth. This mistaken sort of action leads directly to that absurd proposition: one religion is just as good as another, for there is no distinction here between truth and falsehood.  “This attitude”, to quote Pope Leo again, “is directed to the destruction of all religions, but particularly the Catholic faith, which cannot be placed on a level with other religions without serious injustice, since it alone is true”. Moreover, to contend that there is nothing to choose between contradictories and among contraries can lead only to this fatal conclusion: a reluctance to accept any religion either in theory or practice. (John XXIII. Encyclical Ad Petri cathedram, n.17, June 29, 1959)

And here, beloved Sons and Venerable Brothers, We should mention again and censure a very grave error in which some Catholics are unhappily engaged, who believe that men living in error, and separated from the true faith and from Catholic unity, can attain eternal life. Indeed, this is certainly quite contrary to Catholic teaching. It is known to Us and to you that they who labor in invincible ignorance of our most holy religion and who, zealously keeping the natural law and its precepts engraved in the hearts of all by God, and being ready to obey God, live an honest and upright life, can, by the operating power of divine light and grace. […] But, the Catholic dogma that no one can be saved outside the Catholic Church is well-known; and also that those who are obstinate toward the authority and definitions of the same Church, and who persistently separate themselves from the unity of the Church, and from the Roman Pontiff, the successor of Peter, to whom “the guardianship of the vine has been entrusted by the Savior”. (Pius IX, Encyclical Quanto conficiamur moerore, August 10, 1863)

In these times of confusion and disorder, it is not unusual to see Christians, Catholics – even within the secular clergy and cloisters – who constantly have a word of conformity, of conciliation and negotiation on their lips. Very well! I do not hesitate to declare: these men are in error, and do not consider them to be the lesser enemies of the Church. We live in a corrupt and pestilent atmosphere and we must know how to preserve ourselves from it. Let us not allow ourselves to be contaminated by false doctrines, which lose all things under the pretext of saving all. (Pius IX, Speech in the Church of Aracoeli, September 17, 1861)

For since they hold it for certain that men destitute of all religious sense are very rarely to be found, they seem to have founded on that belief a hope that the nations, although they differ among themselves in certain religious matters, will without much difficulty come to agree as brethren in professing certain doctrines, which form as it were a common basis of the spiritual life. For which reason conventions, meetings and addresses are frequently arranged by these persons, at which a large number of listeners are present, and at which all without distinction are invited to join in the discussion, both infidels of every kind, and Christians, even those who have unhappily fallen away from Christ or who with obstinacy and pertinacity deny His divine nature and mission. Certainly such attempts can nowise be approved by Catholics, founded as they are on that false opinion which considers all religions to be more or less good and praiseworthy, since they all in different ways manifest and signify that sense which is inborn in us all, and by which we are led to God and to the obedient acknowledgment of His rule. Not only are those who hold this opinion in error and deceived, but also in distorting the idea of true religion they reject it, and little by little, turn aside to naturalism and atheism, as it is called; from which it clearly follows that one who supports those who hold these theories and attempt to realize them, is altogether abandoning the divinely revealed religion. (Pius XI. Encyclical Mortalium animos, January 6, 1928)

Therefore the Church announces the good tidings of salvation to those who do not believe, so that all men may know the true God and Jesus Christ whom He has sent, and may be converted from their ways, doing penance. (Vatican Council II. Constitution Sacrosanctum concilium, n.9, December 4, 1963)

We have committed to the protection and guidance of heaven those who do not belong to the visible Body of the Catholic Church, solemnly declaring that after the example of the Good Shepherd We desire nothing more ardently than that they may have life and have it more abundantly. Imploring the prayers of the whole Church We wish to repeat this solemn declaration in this Encyclical Letter in which We have proclaimed the praises of the “great and glorious Body of Christ” and from a heart overflowing with love We ask each and every one of them to correspond to the interior movements of grace, and to seek to withdraw from that state in which they cannot be sure of their salvation. (Pius XII. Encyclical Mystici Corporis Christi, June 29, 1943)

 “Have mercy on me, O Lord, for I am languishing; my eye wastes away because of grief.” If you suffer, why are you angry? Your anger is due to sins of others. Who would not be enraged, observing those who confess God but deny him through their conduct? Who would not be enraged seeing those who renounce the world with their words but not with their deeds? Who would not become angry observing how some betray their brothers, unfaithful to the kiss of peace they exchanged during the  celebration of the divine Sacraments? Who may number all of the causes of anger within the body of Christ, where interiorly lives the Spirit of Christ, and which moans as the grain amid the straw. Truly, it is difficult to find those who moan in this way, those who become angry with this anger, as it is difficult to see the grain while threshing the crops. He who doesn’t know how much grain was sown, thinks that all is straw. Yet, from what he believes is all straw will follow the purification of a great number. These, precisely, who do not moan and grieve are the cause of the anger of the one who said in another place “Zeal for your house has consumed me.” (Ps 69: 9) Seeing the number of those who practice evil works, another place says:  “Burning indignation seizes me because of the wicked, who forsake your law.” (Ps 119: 53) And further on: “I look at the faithless with disgust, because they do not keep your commandments.” (Ps 119: 158) (Saint Augustine. Exposition of Psalm 30, Sermon 2, n.4)