Reason is not omnicompetent. It cannot solve all problems. One problem it cannot solve for us has to do with helping us attain to our end.
It was necessary for man’s salvation that there should be a knowledge revealed by God besides philosophical science built up by human reason. Firstly, indeed, because man is directed to God, as to an end that surpasses the grasp of his reason: “The eye hath not seen, O God, besides Thee, what things Thou hast prepared for them that wait for Thee” (Isaiah 64:4). But the end must first be known by men who are to direct their thoughts and actions to the end. Hence it was necessary for the salvation of man that certain truths which exceed human reason should be made known to him by divine revelation. Even as regards those truths about God which human reason could have discovered, it was necessary that man should be taught by a divine revelation; because the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors. Whereas man’s whole salvation, which is in God, depends upon the knowledge of this truth. Therefore, in order that the salvation of men might be brought about more fitly and more surely, it was necessary that they should be taught divine truths by divine revelation. It was therefore necessary that besides philosophical science built up by reason, there should be a sacred science learned through revelation. [ST Ia q1 a1]
The Summa Theologiae isn’t an apologetics work. It’s a theological one that presumes its reader is a student with what was considered at the time to be adequate preparation for tackling it. It would be foolish to get on Aquinas’ case for the fact that he doesn’t prove the principles which underlie what he has said here. He does that in another work: the Summa Contra Gentiles. With that in mind, a few observations can be made.
First: it’s impossible for man to save himself. This is a subject to which Aquinas will return again and again, but it’s already implicit here: you can’t attain salvation on your own if you are incapable of knowing on your own what you have to know in order to be saved. It’s not possible. Consequently we start to see in the very beginning of St Thomas’ magnum opus the contradiction of criticisms some Protestants make against the Catholic Church: we cannot save ourselves by anything we do.
Why does he appeal to reason here rather than to things that we more ordinarily consider to be things that we do (like good works)? We can see traces of the answer to that in the opening of the SCG:
Now, the end of each thing is that which is intended by its first author or mover. But the first author and mover of the universe is an intellect, as will be later shown. The ultimate end of the universe must, therefore, be the good of an intellect. This good is truth. Truth must consequently be the ultimate end of the whole universe, and the consideration of the wise man aims principally at truth. So it is that, according to His own statement, divine Wisdom testifies that He has assumed flesh and come into the world in order to make the truth known: “For this was I born, and for this came I into the world, that I should give testimony to the truth” (John 18:37). The Philosopher himself establishes that first philosophy is the science of truth, not of any truth, but of that truth which is the origin of all truth, namely, which belongs to the first principle whereby all things are. The truth belonging to such a principle is, clearly, the source of all truth; for things have the same disposition in truth as in being. [I:1]
He appeals in ST I q1 a1 to reason’s place in our salvation because “truth must consequently be the ultimate end of the whole universe” and it is man’s capacity for reason that suits him for discovering truth. But as he shows reason can’t attain to all truth, and that is why we need those truths that God has made known to us by means of revelation.
So far so good, but how do we discover what truths He has revealed? For that we need an authorized teacher. We can’t reliably do it in any other way, and if we reject that authorized teacher, then in the end what we’re doing is attempting to make reason do things it can’t.