6 Thus then He spoke to the Jews, and added, “And He that sent me is with me.” He had already said this also before, but of this important point He is constantly reminding them—“He sent me,” and “He is with me.” If then, O Lord, He is with You, not so much has the One been sent by the other, but you Both have come. And yet, while Both are together, One was sent, the Other was the sender; for incarnation is a sending, and the incarnation itself belongs only to the Son and not to the Father.
The Father therefore sent the Son, but did not withdraw from the Son. For it was not that the Father was absent from the place to which He sent the Son. For where is not the Maker of all things? Where is He not, who said, “I fill heaven and earth”? But perhaps the Father is everywhere, and the Son not so? Listen to the evangelist: “He was in this world, and the world was made by Him.” Therefore said He, “He that sent me,” by whose power as Father I am incarnate, “is with me—has not left me.”
Why has He not left me? “He has not left me,” He says, “alone; for I do always those things that please Him.” That equality exists always; not from a certain beginning, and then onwards; but without beginning, without end. For Divine generation has no beginning in time, since time itself was created by the Only-begotten.
Source: Tractates on the Gospel of John (New Advent)