15 “It is,” then, said He, “my Father that glorifies me; of whom you say, that He is your God: and you have not known Him.” See, my brethren, how He shows that God Himself is the Father of the Christ, who was announced also to the Jews. I say so for this reason, that now again there are certain heretics who say that the God revealed in the Old Testament is not the Father of Christ, but some prince or other, I know not what, of evil angels. There are Manicheans who say so; there are Marcionites who say so.
There are also, perhaps, other heretics, whom it is either unnecessary to mention, or all of whom I cannot at present recall; yet there have not been wanting those who said this. Attend, then, that you may have something also to affirm against such. Christ the Lord calls Him His Father whom they called their God, and did not know; for had they known [that God] Himself they would have received His Son. “But I,” said He, “know Him.” To those judging after the flesh He might have seemed from such words to be self-assuming, because He said, “I know Him.”
But see what follows: “If I should say that I know Him not, I shall be a liar like you.” Let not, then, self-assumption be so guarded against as to cause the relinquishment of truth. “But I know Him, and keep His saying.” The saying of the Father He was speaking as Son; and He Himself was the Word of the Father, that was speaking to men.
Source: Tractates on the Gospel of John (New Advent)