There is more to the Catholic Faith than may be encompassed by the rational powers of man. Consequently it is not merely a philosophy.
It is written (2 Timothy 3:16): “All Scripture, inspired of God is profitable to teach, to reprove, to correct, to instruct in justice.” Now Scripture, inspired of God, is no part of philosophical science, which has been built up by human reason. Therefore it is useful that besides philosophical science, there should be other knowledge, i.e. inspired of God. [ST I, Q1, A1]
That’s not the strongest argument that St. Thomas will make for this, but it is the first appeal to authority that he makes in the Summa.
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. [ibid.]
Because our end is one that is beyond our abilities to attain or apprehend, it was necessary for God to reveal such things to us. But it was also good for God to reveal to us even things that we might otherwise have been able to deduce by means of reason, because attaining knowledge by way of our natural powers is difficult and prone to error. As Aristotle says,
The investigation of the truth is in one way hard, in another easy. An indication of this is found in the fact that no one is able to attain the truth adequately, while, on the other hand, we do not collectively fail, but every one says something true about the nature of things, and while individually we contribute little or nothing to the truth, by the union of all a considerable amount is amassed. Therefore, since the truth seems to be like the proverbial door, which no one can fail to hit, in this respect it must be easy, but the fact that we can have a whole truth and not the particular part we aim at shows the difficulty of it.
Perhaps, too, as difficulties are of two kinds, the cause of the present difficulty is not in the facts but in us. For as the eyes of bats are to the blaze of day, so is the reason in our soul to the things which are by nature most evident of all. [Metaphysics II, 1 (993b1-11)]
And St. Thomas:
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. [ST, loc. cit.]
We can infer the reason for the mixture of errors from what Aristotle says; but why would these truths be known only by a few if we were limited only to what reason can discover? Because men differ in their intellectual gifts; some are capable of understanding things that the rest of us simply cannot grasp because of our own limitations: “He who has the superior intellect understands many things that the other cannot grasp at all. Such is the case with a very simple person who cannot at all grasp the subtle speculations of philosophy” (St. Thomas, Summa Contra Gentiles, I, 3, 3). But all men need to be able to attain to their end, which is God. Since all men need knowledge of divine truth in some capacity, but since not all can discover it on their own, and since some things we need to know cannot be discovered by reason at all, we need divine revelation.
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, loc. cit.]
Hence we can see that the Catholic Faith is in no way rationalist. It cannot be measured by the mind of man, because some things cannot be comprehended by man at all. Such things must be received by faith.
It might be worth pointing out, however, that there is a certain way in which Protestantism most certainly is a rationalistic faith. The Protestant claims that his understanding of divine truth is obtained directly from the Bible. But he is limited in this by his own intellectual capacity: that is, the truth that he will perceive in the Bible will be only that which he is capable of grasping himself. Man becomes the measure of divine revelation; the Truth is reduced to that which the man is able to see himself. The Catholic Faith is not like this.