9 How, then, is His judgment true, but because the Son is true? For this He said: “And if I judge, my judgment is true; because I am not alone, but I and the Father that sent me.” Just as if He had said, “My judgment is true,” because I am the Son of God. How dost Thou prove that You are the Son of God? “Because I am not alone, but I and the Father that sent me.” Blush, Sabellian; you hear the Son, you hear the Father. Father is Father, Son is Son.
He said not, I am the Father, and I the same am the Son; but He says, “I am not alone.” Why are You not alone? Because the Father is with me. “I am, and the Father that sent me;” you hear, “I am, and He that sent me.” Lest thou lose sight of the person, distinguish the persons. Distin guish by understanding, do not separate by faithlessness; lest again, fleeing as it were Charybdis, thou rush upon Scylla. For the whirlpool of the impiety of the Sabellians was swallowing you, to say that the Father is the same who is Son: just now you have learned, “I am not alone, but I and the Father that sent me.”
You acknowledge that the Father is Father, and that the Son is Son; you acknowledge rightly. But do not say the Father is greater, the Son is less; do not say, the Father is gold, the Son is silver. There is one substance, one Godhead, one co-eternity, perfect equality, no unlikeness. For if you only believe that Christ is another, not the same person that the Father is, but yet imagine that in respect of His nature He is somewhat different from the Father, you have indeed escaped Charybdis, but you have been wrecked on the rocks of Scylla.
Steer the middle course, avoid each of the two perilous sides. Father is Father, Son is Son. You say now, Father is Father, Son is Son: you have fortunately escaped the danger of the absorbing whirl; why would you go unto the other side to say, the Father is this, the Son that? The Son is another person than the Father is, this you say rightly; but that He is different in nature, you say not rightly. Certainly the Son is another person, because He is not the same who is Father and the Father is another person, because He is not the same who is Son: nevertheless, they are not different in nature, but the selfsame is both Father and Son.
What means the self-same? God is one. You have heard, “Because I am not alone, but I and the Father that sent me:” hear how you may believe Father and Son; hear the Son Himself, “I and the Father are one.” He said not, I am the Father; or, I and the Father is one person; but when He says, “I and the Father are one,” hear both, both the one, unum, and the are, sumus, and you shall be delivered both from Charybdis and from Scylla. In these two words, in that He said one, He delivers you from Arius; in that He said are, He delivers you from Sabellius.
If one, therefore not diverse; if are, therefore both Father and Son. For He would not say are of one person; but, on the other hand, He would not say one of diverse. Hence the reason why He says, “my judgment is true,” is, that you may hear it briefly, because I am the Son of God. But I would have you in such wise believe that I am the Son of God, that you may understand that the Father is with me: I am not Son in such manner as to have left Him; I am not in such manner here that I should not be with Him; nor is He in such manner there as not to be with me: I have taken to me the form of a servant, yet have I not lost the form of God; therefore He says, “I am not alone, but I and the Father that sent me.”
Source: Tractates on the Gospel of John (New Advent)