29 But He immediately adds, For I am from Him, and He has sent Me, to debar heresy from the violent assumption that His being from God dates from the time of His Advent. The Gospel revelation of the mystery proceeds in a logical sequence; first He is born, then He is sent. Similarly, in the previous declaration, we were told of ignorance, first as to Who He is, and then as to whence He is. For the words, I am from Him, and He has sent Me, contain two separate statements, as also do the words, You neither know Me, nor know ye whence I am.
Every man is born in the flesh; yet does not universal consciousness make every man spring from God? How then can Christ assert that either He, or the source of His being, is unknown? He can only do so by assigning His immediate parentage to the ultimate Author of existence; and, when He has done this, He can demonstrate their ignorance of God by their ignorance of the fact that He is the Son of God. Let the victims of this wretched delusion reflect upon the words, You neither know Me, nor know ye whence I am.
All things, they argue, are from nothing; they allow of no exception. They even dare to misrepresent God the Only-begotten as sprung from nothing. How can we explain this ignorance of Christ, and of the origin of Christ, on the part of the blasphemers? The very fact that, as the Scripture says, they know not whence He is, is an indication of that unknowable origin from which He springs. If we can say of a thing that it came into existence out of nothing, then we are not ignorant of its origin; we know that it was made out of nothing, and this is a piece of definite knowledge.
Now He Who came is not the Author of His own being; but He Who sent Him is true, Whom the blasphemers know not. He it was Who sent Him; and they know not that He was the Sender. Thus the Sent is from the Sender; from Him Whom they know not as His Author. The reason why they know not Who Christ is, is that they know not from Whom He is. None can confess the Son who denies that He was born; none can understand that He was born who has formed the opinion that He is from nothing. And indeed He is so far from being made out of nothing, that the heretics cannot tell whence He is.
Source: On the Trinity (New Advent)