Objection 1. It seems that by grace a higher knowledge of God is not obtained than by natural reason. For Dionysius says (De Mystica Theol. i) that whoever is the more united to God in this life, is united to Him as to one entirely unknown. He says the same of Moses, who nevertheless obtained a certain excellence by the knowledge conferred by grace. But to be united to God while ignoring of Him "what He is," comes about also by natural reason. Therefore God is not more known to us by grace than by natural reason.
Objection 2. Further, we can acquire the knowledge of divine things by natural reason only through the imagination; and the same applies to the knowledge given by grace. For Dionysius says (Coel. Hier. i) that "it is impossible for the divine ray to shine upon us except as screened round about by the many colored sacred veils." Therefore we cannot know God more fully by grace than by natural reason.
Objection 3. Further, our intellect adheres to God by grace of faith. But faith does not seem to be knowledge; for Gregory says (Hom. xxvi in Ev.) that "things not seen are the objects of faith, and not of knowledge." Therefore there is not given to us a more excellent knowledge of God by grace.
On the contrary, The Apostle says that "God hath revealed to us His spirit," what "none of the princes of this world knew" (1 Corinthians 2:10), namely, the philosophers, as the gloss expounds.
I answer that, We have a more perfect knowledge of God by grace than by natural reason. Which is proved thus. The knowledge which we have by natural reason contains two things: images derived from the sensible objects; and the natural intelligible light, enabling us to abstract from them intelligible conceptions.
Now in both of these, human knowledge is assisted by the revelation of grace. For the intellect's natural light is strengthened by the infusion of gratuitous light; and sometimes also the images in the human imagination are divinely formed, so as to express divine things better than those do which we receive from sensible objects, as appears in prophetic visions; while sometimes sensible things, or even voices, are divinely formed to express some divine meaning; as in the Baptism, the Holy Ghost was seen in the shape of a dove, and the voice of the Father was heard, "This is My beloved Son" (Matthew 3:17).
Reply to Objection 1. Although by the revelation of grace in this life we cannot know of God "what He is," and thus are united to Him as to one unknown; still we know Him more fully according as many and more excellent of His effects are demonstrated to us, and according as we attribute to Him some things known by divine revelation, to which natural reason cannot reach, as, for instance, that God is Three and One.
Reply to Objection 2. From the images either received from sense in the natural order, or divinely formed in the imagination, we have so much the more excellent intellectual knowledge, the stronger the intelligible light is in man; and thus through the revelation given by the images a fuller knowledge is received by the infusion of the divine light.
Reply to Objection 3. Faith is a kind of knowledge, inasmuch as the intellect is determined by faith to some knowable object. But this determination to one object does not proceed from the vision of the believer, but from the vision of Him who is believed. Thus as far as faith falls short of vision, it falls short of the knowledge which belongs to science, for science determines the intellect to one object by the vision and understanding of first principles.
Source: Summa Theologica (New Advent)