Reason has its limits. Recently I wrote about intellectual modesty, and this post is certainly related to that. In Summa Contra Gentiles I:5, St Thomas writes that it is fitting for truths beyond the powers of reason to be proposed for our belief. One reason that he gives is that this is a stay against intellectual pride:
 Another benefit that comes from the revelation to men of truths that exceed the reason is the curbing of presumption, which is the mother of error. For there are some who have such a presumptuous opinion of their own ability that they deem themselves able to measure the nature of everything; I mean to say that, in their estimation, everything is true that seems to them so, and everything is false that does not. So that the human mind, therefore, might be freed from this presumption and come to a humble inquiry after truth, it was necessary that some things should be proposed to man by God that would completely surpass his intellect. [emphasis added]
In short: revelation acts as a curb against the intellectual vice of humanism. Related to this is something that he writes in Summa Theologiae Ia q1 a6 ad 2, where he says this about the science of sacred doctrine:
The principles of other sciences either are evident and cannot be proved, or are proved by natural reason through some other science. But the knowledge proper to this science comes through revelation and not through natural reason. Therefore it has no concern to prove the principles of other sciences, but only to judge of them. Whatsoever is found in other sciences contrary to any truth of this science must be condemned as false: “Destroying counsels and every height that exalteth itself against the knowledge of God” (2 Corinthians 10:4-5). [emphasis added]
The sort of intellectual humility that he implicitly endorses in these two passages is something that I have to learn, and in beginning to learn it I was inexorably led to become Catholic. Consider the following from the Westminster Confession of Faith (a Presbyterian doctrinal standard):
All things in Scripture are not alike plain in themselves, nor alike clear unto all: yet those things which are necessary to be known, believed, and observed for salvation are so clearly propounded, and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them. [I:vii]
It was easy for me, particularly with my educational background and an overly healthy sense of my own brain power, to conclude from this that I was very well qualified to make judgments about what the Bible teaches. But given that this is what I thought I could do, in what did my judgments consist? They consisted in this: everything is true that seemed to me so, and everything is false that does not. But this is precisely what Aquinas rejects in the first passage quoted above. Is this not humanism? And is this not precisely what WCF I:vii proposes—namely, that a man needs no guide for understanding what the Bible teaches but rather he can discern the essentials for himself? It’s easy to see, isn’t it, how such a principle would lead a man to say that his church is wrong if he disagrees with them? But this is wrong-headed. If I find myself in disagreement with the Church, then I am the mistaken one.
Intellectual pride is just that: pride. And we must set it aside.