John 8:18

"I am one that bear witness of myself, and the Father that sent me beareth witness of me." Therefore, because it is written in your law that the witness of two men is true, receive our witness, lest ye feel our judgment. "For I," saith He, "judge not any man; but I bear witness of myself:" I defer judgment, I defer not the witness. Let us, brethren, choose for ourselves God as our judge, God as our witness, against the tongues of men, against the weak suspicions of mankind. For He who is the judge disdains not to be witness, nor is He advanced in honor when He becomes judge; since He who is witness will also Himself be judge. In what way is He witness? Because He asks not another to learn from Him who thou art. In what way is He judge? Because He has the power of killing and making alive, of condemning and acquitting, of casting down into hell and of raising up into heaven, of joining to the devil and of crowning with the angels. Since, therefore, He has this power, He is judge. Now, because He requires not another witness that He may know thee; and that He who will hereafter judge thee is now seeing thee, there is no means whereby thou canst deceive Him when He begins to judge. For there is no furnishing thyself with false witnesses who can circumvent that judge when He shall begin to judge thee. This is what God says to thee: When thou despisedst, I did see it; and when thou believedst not, I did not frustrate my sentence. I delayed it, not removed it. Thou wouldst not hear what I enjoined, thou shalt feel what I foretold. But if thou hearest what I enjoined, thou shalt not feel the evils which I have foretold, but thou shalt enjoy the good things which I have promised.

Blush, Sabellian; thou hearest the Son, thou hearest the Father. Father is Father, Son is Son. He said not, I am the Father, and I the same am the Son; but He saith, "I am not alone." Why art Thou not alone? Because the Father is with me. "I am, and the Father that sent me;" thou hearest, "I am, and He that sent me." Lest thou lose sight of the person, distinguish the persons. Distinguish 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 thee, to say that the Father is the same who is Son: just now thou hast learned, "I am not alone, but I and the Father that sent me." Thou dost acknowledge that the Father is Father, and that the Son is Son thou dost rightly acknowledge: 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 thou 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, thou hast indeed escaped Charybdis, but thou hast been wrecked on the rocks of Scylla. Steer the middle course, avoid each of the two perilous sides. Father is Father, Son is Son. Thou sayest now, Father is Father, Son is Son: thou hast fortunately escaped the danger of the absorbing whirl; why wouldst thou go unto the other side to say, the Father is this, the Son that? The Son is another person than the Father is, this thou sayest rightly; but that He is different in nature, thou sayest not rightly. […]

But that thou mayest understand how that the Father is with Me, it is not for the Son ever to leave the Father. I have taken up the form of a servant; but I have not lost the form of God. He had spoken of judgment: now He speaks of witness: It is also written in your law, that the testimony of two men is true. (Tract. xxxvi. 10) There is much difficulty, and a great mystery seems to be contained, in God's words, In the mouth of two or three witnesses, let every word be established. (Deut. 10) It is possible that two may speak false. The chaste Susannah was arraigned by two false witnesses: the whole people spake against Christ falsely. How then must we understand the word, By the mouth of two or three witnesses shall every word be established: except as an intimation of the mystery of the Trinity, in which is perpetual stability of truth? Receive then our testimony, lest ye feel our judgment. I delay My judgment: I delay not My testimony: I am one that beareth witnes of Myself, and the Father that sent Me beareth witness of Me.

What would the heretics say here? (They would say,) "How is he better than man, if we take what he hath said simply? For this rule is laid down in the case of men, because no man by himself is trustworthy. But in the case of God, how can one endure such a mode of speaking? How then is the word 'two' used? Is it because they are two, or because being men they are therefore two? If it is because they are two, why did he not betake himself to John, and say, I bear witness of myself, and John beareth witness of me? Wherefore not to the angels? Wherefore not to the prophets? For he might have found ten thousand other testimonies." But he desireth to show not this only that there are Two, but also that they are of the same Substance.

"I am One that bear witness of Myself; and the Father that sent Me beareth witness of Me." Had He been of inferior substance, He would not have put this. But now that thou mayest not deem that the Father is included, to make up the number (of two), observe that His power hath nothing different (from the Father's). A man bears witness when he is trustworthy of himself, not when he himself needs testimony, and that too in a matter pertaining to another; but in a matter of his own, where he needs the witness of another, he is not trustworthy. But in this case it is all contrary. For He though bearing witness in a matter of His own, and saying that He is borne witness to by another, asserteth that He is trustworthy, in every way manifesting His independence.