Thomas on negative theology

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. (ST, 1a q12 a13 ad1)