28 And indeed the Son never makes for Himself a lower claim than is contained in this designation, given Him by His Father. The Father's words, This is My Son, reveal His nature; those which follow, Hear Him, are a summons to us to listen to the mystery and the faith which He came down from heaven to bring; to learn that, if we would be saved, our confession must be a copy of His teaching. And in like manner the Son Himself teaches us, in words of His own, that He was truly born and truly came— You neither know Me, nor know ye whence I am, for I am not come of Myself, but He that sent Me is true, Whom ye know not, but I know Him, for I am from Him, and He has sent Me. No man knows the Father; the Son often assures us of this.
The reason why He says that none knows Him but Himself, is that He is from the Father. Is it, I ask, as the result of an act of creation, or of a genuine birth, that He is from Him? If it be an act of creation, then all created things are from God. How then is it that none of them know the Father, when the Son says that the reason why He has this knowledge is that He is from Him? If He be created, not born, we shall observe in Him a resemblance to other beings who are from God.
Since all, on this supposition, are from God, why is He not as ignorant of the Father as are the others? But if this knowledge of the Father be peculiar to Him, Who is from the Father, must not this circumstance also, that He is from the Father, be peculiar to Him? That is, must He not be the true Son born from the nature of God? For the reason why He alone knows God is that He alone is from God. You observe, then, a knowledge, which is peculiar to Himself, resulting from a birth which also is peculiar to Himself. You recognise that it is not by an act of creative power, but through a true birth, that He is from the Father; and that this is why He alone knows the Father, Who is unknown to all other beings which are from Him.
Source: On the Trinity (New Advent)