Some things can be known with greater confidence than others. For example, I can’t know for sure why my brother hit me with a tennis racket 30 years ago, although it’s reasonable to assume that I must have provoked his wrath. :-) St Thomas affirms at the start of the Summa Contra Gentiles that our certainty about things necessarily varies.
[1] The way of making truth known is not always the same, and, as the Philosopher has very well said, “it belongs to an educated man to seek such certitude in each thing as the nature of that thing allows” [I, 3, 1]
This sort of thing is hard to do, just because sometimes it’s difficult to know how much certainty is warranted, and to properly judge our own knowledge of things. This is one reason that St Thomas says (as we have seen previously):
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.
Among other things, these facts call for a particular kind of modesty: we should be cautious in our declarations of certainty when it comes to the exercise of reason. It’s an intellectual vice—it is humanism—to suppose either that all truth is discernible by human reason, or for a man to suppose that he cannot possibly be wrong in any of his reasoning. The latter ought to be obvious just from the nature of the case: nobody’s perfect! The error of the former is seen in what St Thomas goes on to say in SCG I, 3 (see link above):
[3] That there are certain truths about God that totally surpass man’s ability appears with the greatest evidence. Since, indeed, the principle of all knowledge that the reason perceives about some thing is the understanding of the very substance of that being (for according to Aristotle “what a thing is” is the principle of demonstration) [Posterior Analytics II, 3], it is necessary that the way in which we understand the substance of a thing determines the way in which we know what belongs to it. Hence, if the human intellect comprehends the substance of some thing, for example, that of a stone or of a triangle, no intelligible characteristic belonging to that thing surpasses the grasp of the human reason. But this does not happen to us in the case of God. For the human intellect is not able to reach a comprehension of the divine substance through its natural power. For, according to its manner of knowing in the present life, the intellect depends on the sense for the origin of knowledge; and so those things that do not fall under the senses cannot be grasped by the human intellect except in so far as the knowledge of them is gathered from sensible things. Now, sensible things cannot lead the human intellect to the point of seeing in them the nature of the divine substance; for sensible things are effects that fall short of the power of their cause. Yet, beginning with sensible things, our intellect is led to the point of knowing about God that He exists, and other such characteristics that must be attributed to the First Principle. There are, consequently, some intelligible truths about God that are open to the human reason; but there are others that absolutely surpass its power.
This being so, “it is the acme of stupidity for a man to suspect as false what is divinely revealed through the ministry of the angels simply because it cannot be investigated by reason” (ibid.).
In short: we should be cautious both to say too much about ourselves, or to say too little about the certainty or authority of revelation. There’s an obvious intersect when it comes to understanding the Bible: it’s the Protestant mistake to suppose that I am entirely capable of interpreting the Scriptures without error, or to think that if I disagree with what the Church teaches then I must be right and the Church must be wrong.
Aquinas, Etc. 
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