St Thomas Aquinas on Apophaticism

The reason why no created species can represent the divine essence is plain:  For nothing finite can represent the infinite as it is; but every created species is finite; therefore it cannot represent the infinite as it is.  Further, God is his own existence and therefore his wisdom and greatness and anything else are the same.  But all those cannot be represented through one created thing.  Therefore, the knowledge by which God is seen through creatures is not a knowledge of his essence, but a knowledge that is dark and mirrored, and from afar.  Everyone sees him, in one of the above ways [e.g. "through a created substitute presented to the bodily sight"; "through a representation in the imagination"; "through an intelligible species abstracted from material things"], from afar (Job 36:25), because we do not know what God is by all these acts of knowing, but what he is not, or that he is.  Hence Denys says, in his Mystical theology, that the perfect way in which God is known in this present life is by taking away all creatures and every thing understood by us.

Super Evangelium S. Ioannis,
C.1, L.11, §211.