The Old Testament attests that God is the source of all truth. His Word is truth (cf. Prov 8:7; 2 Sam 7:28). His Law is truth (cf. Ps 119:90). His faithfulness endures to all generations (Ps 119:142; Lk 1:50). Since God is ‘true’ (Rom 3:4), the members of his people are called to live in the truth (cf. Ps 119:30). In Jesus Christ, the whole of God’s truth has been made manifest. ‘Full of grace and truth’ (Jn 1:14), he came as the ‘light of the world’ (Jn 8:12), he is the Truth (cf. 14:6). ‘Whoever believes in me may not remain in darkness’ (Jn 12:46). The disciple of Jesus continues in his word so as to know ‘the truth [that] will make you free’ (cf. Jn 8:32) and that sanctifies (cf. Jn 17:17). To follow Jesus is to live in ‘the Spirit of truth,’ whom the Father sends in his name and who leads ‘into all the truth’ (Jn 16:13). To his disciples Jesus teaches the unconditional love of truth: ‘Let what you say be simply ‘Yes or No’ (Mt 5:37). (Catechism of the Catholic Church, no. 2465-2466)

The Christian is not to ‘be ashamed then of testifying to our Lord’ (2Tim 1:8). In situations that require witness to the faith, the Christian must profess it without equivocation. (Catechism of the Catholic Church, no. 2471)

But Moses answered the people, ‘Fear not! Stand your ground, and you will see the victory the Lord will win for you today. These Egyptians whom you see today you will never see again. The Lord himself will fight for you; you have only to keep still.’ (Ex 14: 13-14)

‘I will be standing there in front of you on the rock in Horeb. Strike the rock, and the water will flow from it for the people to drink.’ This Moses did, in the presence of the elders of Israel. The place was called Massah and Meribah, because the Israelites quarreled there and tested the Lord, saying, ‘Is the Lord in our midst or not?’ (Ex 17: 6-7)

But if any of you lacks wisdom, he should ask God who gives to all generously and ungrudgingly, and he will be given it. But he should ask in faith, not doubting, for the one who doubts is like a wave of the sea that is driven and tossed about by the wind. For that person must not suppose that he will receive anything from the Lord, since he is a man of two minds, unstable in all his ways. (Jas 1:5-8)

The first commandment requires us to nourish and protect our faith with prudence and vigilance, and to reject everything that is opposed to it. There are various ways of sinning against faith:

Voluntary doubt about the faith disregards or refuses to hold as true what God has revealed and the Church proposes for belief.
Involuntary doubt refers to hesitation in believing, difficulty in overcoming objections connected with the faith, or also anxiety aroused by its obscurity. If deliberately cultivated doubt can lead to spiritual blindness.
Incredulity is the neglect of revealed truth or the willful refusal to assent to it. (Catechism of the Catholic Church, no. 2088-2089)

The eighth commandment forbids misrepresenting the truth in our relations with others. This moral prescription flows from the vocation of the holy people to bear witness to their God who is the truth and wills the truth. Offenses against the truth express by word or deed a refusal to commit oneself to moral uprightness: they are fundamental infidelities to God and, in this sense, they undermine the foundations of the covenant. (Catechism of the Catholic Church, no. 2464)

Faith bears its fruit in love: it means keeping the word and the commandments of Jesus, it means abiding with him in the Father who, in him, so loves us that he abides with us. In this new covenant the certitude that our petitions will be heard is founded on the prayer of Jesus (Cf. Jn 14:13-14). (Catechism of the Catholic Church, no. 2614)

‘All human beings desire to know’, (Aristotle, Metaphysics, I, 1) and truth is the proper object of this desire. Everyday life shows how concerned each of us is to discover for ourselves, beyond mere opinions, how things really are. Within visible creation, man is the only creature who not only is capable of knowing but who knows that he knows, and is therefore interested in the real truth of what he perceives. People cannot be genuinely indifferent to the question of whether what they know is true or not. If they discover that it is false, they reject it; but if they can establish its truth, they feel themselves rewarded. It is this that Saint Augustine teaches when he writes: ‘I have met many who wanted to deceive, but none who wanted to be deceived’ (Confessions, X, 23, 33: CCL 27, 173). (John Paul II. Encyclical Fides et ratio, no. 25, September 14, 1998)

This means that the human being – the one who seeks the truth – is also the one who lives by belief. […] Human perfection, then, consists not simply in acquiring an abstract knowledge of the truth, but in a dynamic relationship of faithful self-giving with others. It is in this faithful self-giving that a person finds a fullness of certainty and security. (John Paul II. Encyclical Fides et ratio, no. 31-32, September 14, 1998)

The truth of life is the truth whereby a thing is true, not whereby a person says what is true. Life like anything else is said to be true, from the fact that it attains its rule and measure, namely, the divine law; since rectitude of life depends on conformity to that law. This truth or rectitude is common to every virtue. (Saint Thomas Aquinas. Summa Theologica, II-II, q. 109, a.2, ad 3)

Because truth is strong in itself and is overcome by no attack, it must be our intention to show that the truth of faith cannot he overcome by reason. (Saint Thomas Aquinas. Suma contra Gentiles, l. IV, c. 10, n. 15)

Genuine freedom is an outstanding manifestation of the divine image in man. For God willed to leave man ‘in the power of his own counsel’ (cf. Sir 15:14), so that he would seek his Creator of his own accord and would freely arrive at full and blessed perfection by cleaving to God (cf. Gaudium et Spes, 17). Although each individual has a right to be respected in his own journey in search of the truth, there exists a prior moral obligation, and a grave one at that, to seek the truth and to adhere to it once it is known (cf. Gaudium et Spes, 11). (John Paul II. Encyclical Veritatis splendor, no. 34, August 6, 1993)

‘All men are bound to seek the truth, especially in what concerns God and his Church, and to embrace it and hold on to it as they come to know it’ (DH 1 no. 2). This duty derives from ‘the very dignity of the human person’ (DH 2 no. 1). (Catechism of the Catholic Church, no. 2104)

Man tends by nature toward the truth. He is obliged to honor and bear witness to it: ‘It is in accordance with their dignity that all men, because they are persons . . . are both impelled by their nature and bound by a moral obligation to seek the truth, especially religious truth. They are also bound to adhere to the truth once they come to know it and direct their whole lives in accordance with the demands of truth.’ (DH 2 no. 2.) (Catechism of the Catholic Church, no. 2467)

I am writing you about these matters, although I hope to visit you soon. But if I should be delayed, you should know how to behave in the household of God, which is the church of the living God, the pillar and foundation of truth. (1Tim 3:14-15)

The light of God’s face shines in all its beauty on the countenance of Jesus Christ, ‘the image of the invisible God’ (Col 1:15), the ‘reflection of God’s glory’ (Heb 1:3), ‘full of grace and truth’ (Jn 1:14). Christ is ‘the way, and the truth, and the life’ (Jn 14:6). Consequently the decisive answer to every one of man’s questions, his religious and moral questions in particular, is given by Jesus Christ […] Jesus Christ, the ‘light of the nations’, shines upon the face of his Church, which he sends forth to the whole world to proclaim the Gospel to every creature (cf. Mk 16:15). (Cf. Lumen Gentium, 1). Hence the Church, as the People of God among the nations, (Cf. ibid., 9). while attentive to the new challenges of history and to mankind’s efforts to discover the meaning of life, offers to everyone the answer which comes from the truth about Jesus Christ and his Gospel. The Church remains deeply conscious of her ‘duty in every age of examining the signs of the times and interpreting them in the light of the Gospel, so that she can offer in a manner appropriate to each generation replies to the continual human questionings on the meaning of this life and the life to come and on how they are related’ (Gaudium et Spes, 4). (John Paul II. Encyclical Veritatis splendor, no. 2, August 6, 1993)

The mission of the Magisterium is linked to the definitive nature of the covenant established by God with his people in Christ. It is this Magisterium’s task to preserve God’s people from deviations and defections and to guarantee them the objective possibility of professing the true faith without error. Thus, the pastoral duty of the Magisterium is aimed at seeing to it that the People of God abides in the truth that liberates. (Catechism of the Catholic Church, no. 890)

Indeed, human reason, lest it be deceived and err in a matter of so great importance, ought to search diligently for the fact of divine revelation so that it can know with certainty that God has spoken, and so render to Him, as the Apostle so wisely teaches, ‘a rational service’ (Rom 12:1). For who does not know, or cannot know that all faith is to be given to God who speaks, and that nothing is more suitable to reason itself than to acquiesce and firmly adhere to those truths which it has been established were revealed by God, who can neither deceive nor be deceived? But, how many, how wonderful, how splendid are the proofs at hand by which human reason ought to be entirely and most clearly convinced that the religion of Christ is divine, and that ‘every principle of our dogmas has received its root from above, from the Lord of the heavens,’ and that, therefore, nothing is more certain than our faith, nothing more secure, that there is nothing more holy and nothing which is supported on firmer principles. (Denzinger-Hünermann 2778- 2779. Pius IX, Encyclical Qui pluribus, November 9, 1846)

We consider another abundant source of the evils with which the Church is afflicted at present: indifferentism. This perverse opinion is spread on all sides by the fraud of the wicked who claim that it is possible to obtain the eternal salvation of the soul by the profession of any kind of religion, as long as morality is maintained. Surely, in so clear a matter, you will drive this deadly error far from the people committed to your care. With the admonition of the apostle that ‘there is one God, one faith, one baptism’ (Eph 4:5), may those fear who contrive the notion that the safe harbor of salvation is open to persons of any religion whatever. They should consider the testimony of Christ Himself that ‘those who are not with Christ are against Him’ (Lk 11:23), and that they disperse unhappily who do not gather with Him. Therefore ‘without a doubt, they will perish forever, unless they hold the Catholic faith whole and inviolate.’ (Gregory XVI. Encyclical Mirari vos, August 15, 1832)

And first of all education belongs preeminently to the Church, by reason of a double title in the supernatural order, conferred exclusively upon her by God Himself; absolutely superior therefore to any other title in the natural order. The first title is founded upon the express mission and supreme authority to teach, given her by her divine Founder: ‘All power is given to me in heaven and in earth. Going therefore teach ye all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost, teaching them to observe all things whatsoever I have commanded you, and behold I am with you all days, even to the consummation of the world’ (Mt 28: 18-20). Upon this magisterial office Christ conferred infallibility, together with the command to teach His doctrine. Hence the Church ‘was set by her divine Author as the pillar and ground of truth, in order to teach the divine Faith to men, and keep whole and inviolate the deposit confided to her; to direct and fashion men, in all their actions individually and socially, to purity of morals and integrity of life, in accordance with revealed doctrine’ (Pius IX, Quum non sine). The second title is the supernatural motherhood, in virtue of which the Church, spotless spouse of Christ, generates, nurtures and educates souls in the divine life of grace, with her Sacraments and her doctrine. (Pius XI. Encyclical Divini Illius magistri, no. 15-17, December 31, 1929)

It is worthy of note how a layman, an excellent writer and at the same time a profound and conscientious thinker, has been able to understand well and express exactly this fundamental Catholic doctrine (cf. A. Manzoni, Osservazioni sulla Morale Cattolica, c. III): ‘The Church does not say that morality belongs purely, in the sense of exclusively, to her; but that it belongs wholly to her. She has never maintained that outside her fold and apart from her teaching, man cannot arrive at any moral truth; she has on the contrary more than once condemned this opinion because it has appeared under more forms than one. She does however say, has said, and will ever say, that because of her institution by Jesus Christ, because of the Holy Ghost sent her in His name by the Father, she alone possesses what she has had immediately from God and can never lose, the whole of moral truth, omnem veritatem, in which all individual moral truths are included, as well those which man may learn by the help of reason, as those which form part of revelation or which may be deduced from it.’ (Pius XI. Encyclical Divini Illius magistri, no. 20, December 31, 1929)

No matter what the Christian does, even in the realm of temporal goods, he cannot ignore the supernatural good. Rather, according to the dictates of Christian philosophy, he must order all things to the ultimate end, namely, the Highest Good. All his actions, insofar as they are morally either good or bad (that is to say, whether they agree or disagree with the natural and divine law), are subject to the judgment and judicial office of the Church. (Pius X. Encyclical Singulari quadam, no. 3, September 24, 1912)

Since therefore we have such proofs, it is not necessary to seek the truth among others which it is easy to obtain from the Church; since the apostles, like a rich man (depositing his money) in a bank, lodged in her hands most copiously all things pertaining to the truth: so that every man, whosoever will, can draw from her the water of life. (Rev 22:17) For she is the entrance to life; all others are thieves and robbers. On this account are we bound to avoid them, but to make choice of the thing pertaining to the Church with the utmost diligence, and to lay hold of the tradition of the truth. (Saint Irenaeus of Lyons. Adversus Haereses, Bk III, c. 4, no.1)

For in the Catholic Church, not to speak of the purest wisdom, to the knowledge of which a few spiritual men attain in this life, so as to know it, in the scantiest measure, indeed, because they are but men, still without any uncertainty (since the rest of the multitude derive their entire security not from acuteness of intellect, but from simplicity of faith,)— not to speak of this wisdom, which you do not believe to be in the Catholic Church, there are many other things which most justly keep me in her bosom. The consent of peoples and nations keeps me in the Church; so does her authority, inaugurated by miracles, nourished by hope, enlarged by love, established by age. The succession of priests keeps me, beginning from the very seat of the Apostle Peter, to whom the Lord, after His resurrection, gave it in charge to feed His sheep, down to the present episcopate. And so, lastly, does the name itself of Catholic, which, not without reason, amid so many heresies, the Church has thus retained; so that, though all heretics wish to be called Catholics, yet when a stranger asks where the Catholic Church meets, no heretic will venture to point to his own chapel or house. Such then in number and importance are the precious ties belonging to the Christian name which keep a believer in the Catholic Church, as it is right they should, though from the slowness of our understanding, or the small attainment of our life, the truth may not yet fully disclose itself. But with you, where there is none of these things to attract or keep me, the promise of truth is the only thing that comes into play. Now if the truth is so clearly proved as to leave no possibility of doubt, it must be set before all the things that keep me in the Catholic Church; but if there is only a promise without any fulfillment, no one shall move me from the faith which binds my mind with ties so many and so strong to the Christian religion. (Saint Augustine of Hippo. Against the ‘Fundamental Epistle’ of Manichaeus, ch.4, no.5)

Hence there exists a close connection and communication between Sacred Tradition and Sacred Scripture. For both of them, flowing from the same divine wellspring, in a certain way merge into a unity and tend toward the same end. For Sacred Scripture is the word of God inasmuch as it is consigned to writing under the inspiration of the divine Spirit, while sacred tradition takes the word of God entrusted by Christ the Lord and the Holy Spirit to the Apostles, and hands it on to their successors in its full purity, so that led by the light of the Spirit of truth, they may in proclaiming it preserve this word of God faithfully, explain it, and make it more widely known. Consequently it is not from Sacred Scripture alone that the Church draws her certainty about everything which has been revealed. Therefore both sacred tradition and Sacred Scripture are to be accepted and venerated with the same sense of loyalty and reverence (cf. Vatican Council I, ‘On Revelation,’ Dz 1786). (Vatican Council II, Dogmatic Constitution Dei verbum, November 18, 1965)