There is a twofold mode of truth in what we profess about God. Some truths about God exceed all the ability of the human reason. Such is the truth that God is triune. But there are some truths which the natural reason also is able to reach. Such are that God exists, that He is one, and the like. In fact, such truths about God have been proved demonstratively by the philosophers, guided by the light of the natural reason.
 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), 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 read 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 truths that absolutely surpass its power.
Aquinas, Summa Contra Gentiles, I.3.2-3. (emphasis mine)