Right off the bat in the Summa Theologiae, St. Thomas rejects rationalism.
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. [I, q.1, a.1]
There are truths that are beyond man’s ability to grasp, and if we are to know them at all they must be revealed to us. Consequently the Christian Faith isn’t rationalist either: man does not define the content of the Faith—of those truths that God reveals to us.
We also see here something that we briefly noticed in a previous post on the opening of the Summa Contra Gentiles: it is difficult for us to attain knowledge. This being the case, God doesn’t merely reveal to us things that we could know in no other way than by revelation; He also declares to us things that we could know by way of reason. Consequently there is some overlap between the things discoverable by reason absolutely speaking and those things we could know only by way of revelation.