Saint Thomas Aquinas is famous for his philosophical proofs for the existence of God, but proving theism does not prove Christianity. Fortunately, Aquinas did not stop there. Although they are less well-known, and sprinkled throughout several of his works, Aquinas extended his apologetic arguments to the truth of Christianity itself.
In many apologetic systems (e.g., McDowell, Geisler, or Montgomery), the key to proving the truthfulness of Christianity (whether or not first proving theism) is arguing for trust in the Bible. In fact, they are usually treated as equivalent. But arguing for Christianity and arguing for the Bible are two different things. The Bible is a product of Christianity, not vice versa.
Such is not Aquinas’s method. He does not argue for trust the Bible per se. One does not find in his apologetic system much in the way of arguments from archaeology, scientific verification, historical reliability, or manuscript transmission. What he did argue for was the truth of the tradition that was the Bible’s source. Not only does this “cut out the middle man,” it also makes better use of miracles as evidence (since using biblical miracles to prove the Bible can become circular if one is not careful).
Miracles in General
Miracles are kind of a hinge between arguments for God and from God. If miracles are acts of God by definition (which Aquinas believes they are), then a miracle would prove God. But it is difficult to make that move if one is not already convinced that a God that can do miracles exists. After all, strange things happen all the time, and without a theistic context, it is easy for modernists to write miracle claims off. Thus, in the classical apologetic method, miracles follow after proofs for God, and take on the function of confirming God’s messengers.
In both of his most famous works (Summa Theologiæ and Summa Contra Gentiles), Aquinas basically switched to discussing Christian doctrine once the existence of God has been demonstrated. However, even though he did not write as elaborately or systematically concerning proofs for the Christian faith, he did not leave the issue hanging. In both Summas, Aquinas asserts that miracles are used to confirm the truth of Christianity:
“The working of miracles is ascribed to faith for two reasons. First, because it is directed to the confirmation of faith, secondly, because it proceeds from God’s omnipotence on which faith relies. Nevertheless, just as besides the grace of faith, the grace of the word is necessary that people may be instructed in the faith, so too is the grace of miracles necessary that people may be confirmed in their faith.” (ST 184.108.40.206.R5).
“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.” (SCG 1.6.1)
The Miracle of the Resurrection
Of the many miracles God has used over the centuries to pick out his spokespeople, the greatest was the resurrection of Jesus Christ. Aquinas says the resurrection of Christ was in fact necessary for coming to faith:
“Secondly, for our instruction in the faith, since our belief in Christ’s Godhead is confirmed by His rising again, because, according to 2 Corinthians 13:4…” (ST 3.53.1)
After dedicating an entire article to various apologetic issues over the value of the resurrection, Aquinas then says,
“Each separate argument [for the risen Christ] would not suffice of itself for showing perfectly Christ’s Resurrection, yet all taken collectively establish it completely, especially owing to the testimonies of the Scriptures, the sayings of the angels, and even Christ’s own assertion supported by miracles.” (ST 3.55.6.R1)
Note again Aquinas’s assertion that miracles support the divine message.
The Miracle of the Church
For Aquinas, miracles include the very existence of the Church. The Christian Church was founded on miraculous claims, and so its genesis is difficult to explain were they absent. Too many people died without recanting their miracles stories for them to have been lies.
“Everything that the Saints believed and handed down to us concerning the faith of Christ is signed with the seal of God. This seal consists of those works which no mere creature could accomplish; they are the miracles by which Christ confirmed the sayings of the apostles and of the Saints.” (“Exposition of the Apostle’s Creed” – Prologue)
Aquinas says the Church remains a verifiable miracle even today. So, even if someone questions miracle stories of old, the Church itself continues to show Christianity’s miraculous nature – for without miracles, ongoing conversion would be even more miraculous:
“If, however, you would say that no one has witnessed these miracles, I would reply in this manner. It is a fact that the entire world worshiped idols and that the faith of Christ was persecuted, as the histories of the pagans also testify. But now all are turned to Christ—wise men and noble and rich—converted by the words of the poor and simple preachers of Christ. Now, this fact was either miracle or it was not. If it is miraculous, you have what you asked for, a visible fact; if it is not, then there could not be a greater miracle than that the whole world should have been converted without miracles. And we need go no further. We are more certain, therefore, in believing the things of faith than those things which can be seen, because God’s knowledge never deceives us, but the visible sense of man is often in error.” (EAC – Prologue)
Aquinas says much the same thing here, noting that so many different kinds of people not only believed, but gave up pleasures in this life for things unseen in the next:
“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. . . .” (SCG 1.6.1)
Thus it is that the kinds of miracles recorded in the Bible need not be repeated:
“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. This wonderful conversion of the world to the Christian faith is the clearest witness of the signs given in the past; so that it is not necessary that they should be further repeated, since they appear most clearly in their effect. For it would be truly more wonderful than all signs if the world had been led by simple and humble men to believe such lofty truths, to accomplish such difficult actions, and to have such high hopes.” (SCG 1.6.1)
Even so, God has not left the world without continuing miraculous activity in His Church:
“Yet it is also a fact that, even in our own time, God does not cease to work miracles through His saints for the confirmation of the faith.” (SCG 1.6.1)
It should also be noted that Aquinas’s use of the Church’s existence and activity as miraculous confirmation of the truth of Christianity actually gains strength as the years go on!
Aquinas was the Church’s best theologian and apologist because he utilized the best philosophy when dealing with matters addressable by philosophy. But Aquinas also used historical evidence when dealing with historical issues. Once showing (via philosophy) that there had to be a single, simple, omnipotent, creator God, Aquinas went on to prove which purported revelation was really from that God.
Because God had to use human means to convey human knowledge – and these means are by definition reproducible by mere humans as counterfeit revelations – God chose to back up the words of his spokesmen with acts that no mere human could produce. These “motives of credibility” are not determining causes that force belief, however. Rather, they are strong enough pointers to the truth that they give a people the intellectual freedom to choose belief without resorting to blind consent.