10 He had spoken of judgment; He means to speak of testimony. “In your law,” says He, “it is written that the testimony of two men is true. I am one that bear witness of myself, and the Father that sent me bears witness of me.” He expounded the law to them also, if they were not unthankful. For it is a great question, my brethren, and to me it certainly appears to have been ordained in a mystery, where God said, “In the mouth of two or three witnesses every word shall stand.” Is truth sought by two witnesses?
Clearly it is; so is the custom of mankind: but yet it may be that even two witnesses lie. The chaste Susanna was pressed by two false witnesses: were they not therefore false because they were two? Do we speak of two or of three? A whole people lied against Christ. If, then, a people, consisting of a great multitude of men, was found a false witness, how is it to be understood that “in the mouth of two or three witnesses every word shall stand,” unless it be that in this manner the Trinity is mysteriously set forth to us, in which is perpetual stability of truth?
Do you wish to have a good cause? Have two or three witnesses,— the Father, Son, and Holy Ghost. In short, when Susanna, the chaste woman and faithful wife, was pressed by two false witnesses, the Trinity supported her in her conscience and in secret: that Trinity raised up from secrecy one witness, Daniel, and convicted the two. 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,” says He, “judge not any man; but I bear witness of myself:” I defer judgment, I defer not the witness.
Source: Tractates on the Gospel of John (New Advent)