Saint Thomas Aquinas: God gave all the divine proofs we needed to believe

In chapter 6 of his Summa Against the Gentiles, written between 1258 and 1265, St. Thomas Aquinas explains that by "accomplishing in a very visible way" works "far above the possibilities of nature"; by inspiring the predictions that the prophets made "long in advance"; and by allowing the "signs, miracles and wonders" that accompanied the testimony of the apostles and are still "today" confirmations reaching us through the saints, God has given all the proofs "that are fitting" to witness, manifest, confirm and guarantee the solidity of the Christian faith. Conversely, because "sect founders" have never "provided supernatural proofs", even though they are "the only ones that can bear proper witness to divine inspiration", all those who give credence to their words "believe lightly".

Le Triomphe de saint Thomas d'Aquin, Francisco de Zurbarán, 1631, Séville, Musée provincial des beaux-arts. / © CC0/wikimedia

Reasons to believe:

The simplicity, coherence and logic of this comprehensive work authored by Saint Thomas Aquinas constitute in itself a strong reason to believe.
The various types of proof of the truth of Christianity that Thomas Aquinas evokes are well known and convincing, starting with the "works far beyond the possibilities of nature" performed in a very public way by Christ, which "testify that the Father has sent him" (Jn 5:36; 10:25). Indeed, when water is changed into wine, loaves and fishes are multiplied, the dead are raised, demons are cast out and the sea and wind are obeying his commands, etc., this would normally cause an awakening of conscience. It was in this sense that Christ reproached the towns on the shores of Lake Tiberias, where he had revealed himself through words and deeds: "Woe to you, Chorazin! Woe to you, Bethsaida! For if the mighty deeds done in your midst had been done in Tyre and Sidon, they would long ago have repented, sitting in sackcloth and ashes. But it will be more tolerable for Tyre and Sidon at the judgment than for you. And as for you, Capernaum, ‘Will you be exalted to heaven? You will go down to the netherworld.For if the mighty deeds done in your midst had been done in Sodom, it would have remained until this day. But I tell you, it will be more tolerable for the land of Sodom on the day of judgment than for you." (Lk 10: 13-15 and Mt 11: 20-24).
The sublimity and wisdom of Christ's words, which pervades the preaching of the apostles and early Christians, and led to the miraculous spread of Christianity despite intense persecutions, is another reason to believe.
So is the fact that God, "long in advance", "foretold" all this "by the mouth of the prophets".
Then there is the fact that the Lord, after having done "works that no one else ever did" (Jn 15:24), also confirmed the words of the apostles and the saints of all times with numerous accompanying "signs, wonders and miracles" (Mk 16:20; Acts 2:22; 2:43; Heb 2:4), which are other very important confirmations.
Taking up and completing all these ideas, Vatican I concluded: "For to the Catholic Church alone belong all those things, so many and so wonderful, which have been Divinely-established for the manifest trustworthiness of the Christian Faith. Of course, even now, the Church by Herself, with Her truly wonderful propagation, Her eminent holiness, and Her inexhaustible fruitfulness in all that is good, with Her Catholic unity and Her invincible stability, is a great and perpetual impetus of credibility, and an irrefutable witness of Her own Divine mission."(Dogmatic Constitution on the Catholic Faith Dei Filius, Chapter 3).

"And not only can faith and reason never be opposed to one another, but they are of mutual aid one to the other. For right reason demonstrates the foundations of faith, and enlightened by its light, cultivates the science of Divine things; while faith frees and guards reason from errors, and furnishes it with manifold knowledge" (Id. Chapter 3).

"If anyone shall say that Divine revelation cannot be made credible by outward signs, and therefore that men ought to be moved to faith solely by the internal experience of each, or by private inspiration; let him be anathema." (Id. Canons III, 3).

"If anyone shall say that miracles can never be known with certainty, and that the divine origin of Christianity cannot be proved by them; let him be anathema." (Ibid).

Pius XII confirmed this by saying: "The many wonderful external signs God has given…are sufficient to prove with certitude by the natural light of reason alone the divine origin of the Christian religion." (Humani Generis, § 4).

Summary:

Here's how St Thomas Aquinas explains the credibility of the Christian faith and denounces the fact of believing "lightly" without the "proper proofs", in that famous chapter 6 of the Summa Against the Gentiles:

"Those who place their faith in this truth, however, “for which the human reason offers no experimental evidence,” do not believe foolishly, as though “following artificial fables” (2 Peter 2:16). For these “secrets of divine Wisdom” (Job 11:6) the divine Wisdom itself, which knows all things to the full, has deigned to reveal to men. It reveals its own presence, as well as the truth of its teaching and inspiration, by fitting arguments; and in order to confirm those truths that exceed natural knowledge, it gives visible manifestation to works that surpass the ability of all nature. Thus, there are the wonderful cures of illnesses, there is the raising of the dead, and the wonderful immutation in the heavenly bodies; and what is more wonderful, there is the inspiration given to human minds, so that simple and untutored persons, filled with the gift of the Holy Spirit, come to possess instantaneously the highest wisdom and the readiest eloquence. When these arguments were examined, through the efficacy of the abovementioned proof, and not the violent assault of arms or the promise of pleasure, and (what is most wonderful of all) in the midst of the tyranny of the persecutors, an innumerable throng of people, both simple and most learned, flocked to the Christian faith. In this faith there are truths preached that surpass every human intellect; the pleasures of the flesh are curbed; it is taught that the things of the world should be spurned. Now, for the minds of mortal men to assent to these things is the greatest of miracles, just as it is a manifest work of divine inspiration that, spurning visible things, men should seek only what is invisible. Now, that this has happened neither without preparation nor by chance, but as a result of the disposition of God, is clear from the fact that through many pronouncements of the ancient prophets God had foretold that He would do this. The books of these prophets are held in veneration among us Christians, since they give witness to our faith.

[...] On the other hand, those who founded sects committed to erroneous doctrines proceeded in a way that is opposite to this, The point is clear in the case of Muhammad. He seduced the people by promises of carnal pleasure to which the concupiscence of the flesh goads us. His teaching also contained precepts that were in conformity with his promises, and he gave free rein to carnal pleasure. In all this, as is not unexpected, he was obeyed by carnal men. As for proofs of the truth of his doctrine, he brought forward only such as could be grasped by the natural ability of anyone with a very modest wisdom. Indeed, the truths that he taught he mingled with many fables and with doctrines of the greatest falsity. He did not bring forth any signs produced in a supernatural way, which alone fittingly gives witness to divine inspiration; for a visible action that can be only divine reveals an invisibly inspired teacher of truth. On the contrary, Muhammad said that he was sent in the power of his arms—which are signs not lacking even to robbers and tyrants. What is more, no wise men, men trained in things divine and human, believed in him from the beginning, Those who believed in him were brutal men and desert wanderers, utterly ignorant of all divine teaching, through whose numbers Muhammad forced others to become his followers by the violence of his arms. Nor do divine pronouncements on the part of preceding prophets offer him any witness. On the contrary, he perverts almost all the testimonies of the Old and New Testaments by making them into fabrications of his own, as can be. seen by anyone who examines his law. It was, therefore, a shrewd decision on his part to forbid his followers to read the Old and New Testaments, lest these books convict him of falsity. It is thus clear that those who place any faith in his words believe foolishly."

The Catholic Church adopted this doctrine of the "Common Doctor" (i.e Aquinas) and the same list of "external proofs" that demonstrate the truth of the Christian faith.

This is summarized, for example, in the Catechism of the Catholic Church, paragraph 156: "So that the submission of our faith might nevertheless be in accordance with reason, God willed that external proofs of his Revelation should be joined to the internal helps of the Holy Spirit." Thus the miracles of Christ and the saints, prophecies, the Church's growth and holiness, and her fruitfulness and stability "are the most certain signs of divine Revelation, adapted to the intelligence of all"; they are "motives of credibility", which show that the assent of faith is "by no means a blind impulse of the mind".

Quotes from Saint Thomas Aquinas

Beyond reasons to believe:

It is not useless to follow the advice the Archangel Raphael gave to Tobias: "God's works must be revealed and celebrated as they deserve" (Tb 12:11). Today, more than ever, this is exactly what we are encouraging all the readers and friends of the 1000 Reasons to Believe project to do: it is important to "publish the wonders of God" (Ps 26:7; 96:3; 145:5) in order to make the great credibility of the Christian faith widely known; this is what we should all strive to do together over the next five years.

Going further:

The Summa Theologica and the complete works of St Thomas Aquinas are available on Aquinas Online.

More information:

Fides et Ratio by Pope John Paul II
Faith, Science, and Reason: Theology on the Cutting Edge by Christopher Baglow (2009)
CNA/EWTN video God: Bridging Science and Faith (CNA/EWTN video about Michel-Yves Bolloré and Olivier Bonnassies's best-seller God: The Science and the Proof not yet published in English)