7 I will accordingly speak; let him who can, understand; and let him who cannot understand, believe: yet will I speak what the Lord says, “You judge after the flesh; I judge not any man,” either now, or after the flesh. “But even, if I judge, my judgment is true.” Why is Your judgment true? “Because I am not alone,” says He, “but I and the Father that sent me.” What then, O Lord Jesus? If You were alone would Your judgment be false: and is it because You are not alone, but Thou and the Father that sent You, that You judge truly?
How shall I answer? Let Himself answer: He says, “My judgment is true.” Why? “Because I am not alone, but I and the Father that sent me.” If He is with You, how has He sent You? And has He sent You, and yet is He also with You? Is it so that having been sent, You have not departed from Him? And You came to us, and yet abode there? How is this to be believed? How apprehended? To these two questions I answer: You say rightly, how is it to be apprehended; how believed, you say not rightly.
Rather, for that reason is it right to believe it, because it is not immediately to be apprehended; for if it were a thing to be immediately apprehended, there would be no need to believe it, because it would be seen. It is because you do not apprehend that you believe, but by believing you are made capable of apprehending. For if you do not believe, you will never apprehend, since you will remain less capable. Let faith then purify you, that understanding may fill you. “My judgment is true,” says He, “because I am not alone, but I and the Father that sent me.”
Therefore, O Lord our God, Jesus Christ, Your sending is Your incarnation. So I see, so I understand: in short, so I believe, in case it may smack of arrogance to say, so I understand. Doubtless the Lord Jesus Christ is even here; rather, was here as to His flesh, is here now as to His Godhead: He was both with the Father and had not left the Father. Hence, in that He is said to have been sent and to have come to us, His incarnation is set forth to us, for the Father did not take flesh.
Source: Tractates on the Gospel of John (New Advent)