If we relied on reason alone, the truth could be known only a few, with much uncertainty, and this after great difficulty. How then, the question may be asked, since all cannot be experts in philosophy or theology anyway are they to know the truth, the full truth, and nothing but the truth, with solid certainty?
Thus the First Vatican Council, which is an absolute must-read for any Catholic wondering about the relationship faith and reason, taught,
Nevertheless, in order that the submission of our faith should be in accordance with reason, it was God's will that there should be linked to the internal assistance of the Holy Spirit external indications of his revelation, that is to say divine acts, and first and foremost miracles and prophecies, which clearly demonstrating as they do the omnipotence and infinite knowledge of God, are the most certain signs of revelation and are suited to the understanding of all.
Hence Moses and the prophets, and especially Christ Our Lord Himself, worked many absolutely clear miracles and delivered prophecies
And a form of this teaching was incorporated into the Oath against Modernism. The Council Fathers wisely foresaw the subtle spread of impiety everywhere, the rise of such doctrines as naturalism and rationalism, and how devastating this environment was gradually going to be to the souls of the Catholic faithful.
Apart from several manifest modern miracles such as those in Lourdes, Fatima and the like, as to the chief prophecies in Holy writ about the Messiah, spoken by Almighty God through His holy Prophets all made and written down several centuries before his advent, only a very few may be mentioned.
The promise of a new covenant, an everlasting one that shall never be broken (Jer 31:31-34;Isa 59:20-21), the institution of a new order of priesthood, according to Melchizedek (Psa 110:4) who had offered bread and wine to God most high (Gen 14:18) and blessed Abraham, the selection of such priests from all nations (Isa 66:21), nations who will be reconciled by the Servant who faces rejection from His former people (Isa 49:5-6), who are henceforth no more His (Dan 9:26), the multitude of who will then offer a pure sacrifice to God in every place throughout the world from sunrise to sunset, while He receives none from the Jews (Mal 1:10-11)
The numerous remarkably clear visions of His Passion, suffering and death for the remission of sins, (Is 52:13-53:12; Wis 2:12-21; Psa 22), His being a Son of Man, who is worshipped from all nations,(Dan 7:13-14) seated at the right hand of God, until His enemies are defeated, (Psa 110:1-2), the pure fountain of baptism present in the new Jerusalem that washes away sins, (Ez 36:25, Zec 13:1) are all clearly foreseen, His divinity manifested, (Mic 5:2, Isa 9:6), and even the very timing of His coming is announced as being a certain number of years after the decree of Artaxerxes and while the temple was yet standing (Dan 9:25-26, Mal 3:1) (which we know historically before 70 A.D and after 62 B.C).
All of this is mentioned to varying degrees even by the early Fathers of the Church in their controversies with pagans. The great medieval Scholastic Doctors went even further in establishing very plainly, from reason alone, simple proofs for the existence of God, or reasons that make it credible.
St.Thomas offers 5 arguments, essentially from causality, cosmology, teleology and morality. All of these have been developed by others since then, and are each very powerful arguments, to anyone inclined toward philosophy. And even someone who accepts the Big bang model unqualified is still forced to admit them.
The Church also enlisted what are called "motives of credibility". Apart from prophesy and miracles, the Church herself is one such sign of the true Faith.
What is more, the Church herself by reason of her astonishing propagation, her outstanding holiness and her inexhaustible fertility in every kind of goodness, by her Catholic unity and her unconquerable stability, is a kind of great and perpetual motive of credibility and an incontrovertible evidence of her own divine mission.
So it comes about that, like a standard lifted up for the nations , she both invites to herself those who have not yet believed, and likewise assures her sons and daughters that the faith they profess rests on the firmest of foundations.