John 8:18

[2.] He applies this to the case in hand (v. 18): I am one that bear witness of myself, and the Father that sent me bears witness of me. Behold two witnesses! Though in human courts, where two witnesses are required, the criminal or candidate is not admitted to be a witness for himself; yet in a matter purely divine, which can be proved only by a divine testimony, and God himself must be the witness, if the formality of two or three witnesses be insisted on, there can be no other than the eternal Father, the eternal Son of the Father, and the eternal Spirit. Now if the testimony of two distinct persons, that are men, and therefore may deceive or be deceived, is conclusive, much more ought the testimony of the Son of God concerning himself, backed with the testimony of his Father concerning him, to command assent; see 1 John 5 7, 9-11. Now this proves not only that the Father and the Son are two distinct persons (for their respective testimonies are here spoken of as the testimonies of two several persons), but that these two are one, not only one in their testimony, but equal in power and glory, and therefore the same in substance. St. Austin here takes occasion to caution his hearers against Sabellianism on the one hand, which confounded the persons in the Godhead, and Arianism on the other, which denied the Godhead of the Son and Spirit. Alius est filius, et alius pater, non tamed aliud, sed hoc ipsum est et pater, et filius, scilicet unus Deus est—The Son is one Person, and the Father is another; they do not, however, constitute two Beings, but the Father is the same Being that the Son is, that is, the only true God. Tract. 36, in Joann. Christ here speaks of himself and the Father as witnesses to the world, giving in evidence to the reason and conscience of the children of men, whom he deals with as men. And these witnesses to the world now will in the great day be witnesses against those that persist in unbelief, and their word will judge men.

Verses 17-20 If we knew Christ better, we should know the Father better. Those become vain in their imaginations concerning God, who will not learn of Christ. Those who know not his glory and grace, know not the Father that sent him. The time of our departure out of the world, depends upon God. Our enemies cannot hasten it any sooner, nor can our friends delay it any longer, than the time appointed of the Father. Every true believer can look up and say with pleasure, My times are in thy hand, and better there than in my own. To all God's purposes there is a time.