4 Lastly, when He had said, “But He that sent me is true, whom you know not,” in order to show them whence they might know that which they did not know, He subjoined, “I know Him.” Therefore seek from me to know Him. But why is it that I know Him? “Because I am from Him, and He sent me.” Gloriously has He shown both. “I am from Him,” said He; because the Son is from the Father, and whatever the Son is, He is of Him whose Son He is. Hence we say that the Lord Jesus is God of God: we do not say that the Father is God of God, but simply God: and we say that the Lord Jesus is Light of Light; we do not say that the Father is Light of Light, but simply Light. Accordingly, to this belongs that which He said “I am from Him.” But as to my being seen of you in the flesh, “He sent me.” When you hear,“He sent me,” do not understand a difference of nature to be meant, but the authority of Him that begets.
Source: Tractates on the Gospel of John (New Advent)