Selection from St. Thomas Aquinas, Super Boetium de Trinitate, Q. III, a. 1
Translated by Rose E. Brennan, S.H.N. (Herder, 1946)
Since the end of human life is beatitude, which consists in the full cognition of divine truths, it is necessary that human life be directed to this beatitude by an initial possession of divine truths by faith, truths which man can hope to know fully in the ultimate state of human perfection.
Certain of these truths that must be known can be attained by reason even in this life: however, although knowledge of them is possible and even possessed by certain men, nevertheless faith is necessary for five reasons, which Rabbi Moses enumerates:
1. First, on account of the depth and subtlety of the matter, by which divine truths are hidden from human understanding. Therefore, lest any man be without some knowledge of them, provision is made that through faith, at least, he know divine truths. Therefore, in Eccles. 7:25 it is said: “It is a great depth, who shall find it out?”
2. Secondly, on account of the weakness of the human intellect from the beginning. For perfection of knowledge does not belong to the human intellect except at the end; therefore, that it should at no time lack a knowledge of God, it requires faith by which it may accept divine truths from the very beginning.
3. Thirdly, because of the many preambles that are required for a knowledge of God according to reason. For this there is needed knowledge of almost all the sciences, since cognition of divine things is the end of them all. But few indeed would comprehend these preambulatory truths or investigate them completely. Therefore, lest large numbers of men should be left without knowledge of divine things, the way of faith has been provided by God Himself.
4. In the fourth place, many men on account of their natural constitution are unfitted for perfect intellectual investigation according to reason; therefore, that these might not lack knowledge of divine truths, the way of faith has been provided.
5. In the fifth place, because of numerous occupations with which men are busied, it would be impossible for all of them to discover, by way of reason, necessary truth in regard to God, and on this account the way of faith has been established, both as regards things that might in some way be known and as regards those that required revelation in order that they be believed.
But in the case of certain divine truths, for a complete understanding of them the human mind in no way suffices, but full knowledge of them is to be awaited in that future life when there will be complete beatitude: such is the truth of the Trinity and the unity of one God; and man is led to knowledge of this, not in accordance with anything due his nature, but by divine grace alone.
Therefore it is necessary that, for a perfection of knowledge of this kind, certain suppositions be proposed which must be believed at first, and from these one is directed into full cognition of those truths which at the outset he held on faith, even as in other sciences also, as has been said. Hence in Is. 7:9 it is said, according to one translation: “Unless thou hadst believed, thou wouldst not understand.” And suppositions of this sort are those that must be believed by all, since in this life they are neither known nor understood by, any one.