John 8:17

It is also written in your Law (Deuteronomy 27:6, Deu 19:5), that the testimony of two men is true : that is to be admitted by the judge, who can base on it a legal sentence, though the testimony may as a matter of fact be false. But a judge must go by the evidence; and so his sentences may be legally right, but in reality wrong. If then the testimony of two men be true, how much more must the sentence of two Divine Persons, the Father and the Son, be accepted as most true, most equitable, and most just? Christ applies this to His own case. For that the Father is with Him, and witnesses to Him, and that He is the Son of the Father, He had more than sufficiently proved, and therefore assumes it. "It is," says Augustine, "a grand and most mysterious question when God says in the mouth of two or three witnesses shall every word be established;' for Susanna was accused by two false witnesses, and all the people witnessed falsely against Christ. But in this way is the Trinity represented as in mystery; for therein is the ever-enduring firmness of truth. If thou wishest to have a good cause, have three witnesses, the Father, the Son, and the Holy Ghost."