If I testify about myself, my testimony is not valid. There is another who testifies in my favor, and I know that his testimony about me is valid
The Pharisees challenged him, "Here you are, appearing as your own witness; your testimony is not valid."
Jesus answered, "Even if I testify on my own behalf, my testimony is valid, for I know where I came from and where I am going. But you have no idea where I come from or where I am going. You judge by human standards; I pass judgment on no one. But if I do judge, my decisions are right, because I am not alone. I stand with the Father, who sent me. In your own Law it is written that the testimony of two men is valid. I am one who testifies for myself; my other witness is the Father, who sent me."
Jesus and the Pharisees are referring to Deuteronomy 19:15: "One witness is not enough to convict a man accused of any crime or offense he may have committed. A matter must be established by the testimony of two or three witnesses."
In the first passage, Jesus is giving the Pharisees reasons for believing that he is the Messiah. He acknowledges that his testimony alone is not valid according to the Law, but adds that there is a second witness for him (God), which makes it valid under the Law. In the second passage, the Pharisees are directly attacking Jesus by saying his testimony is invalid. Jesus responds by saying his testimony is valid, period, because 1) it's true, 2) he came from God and is returning to God (which means that what he says of himself is from God and therefore true, and also that he is doing God's will and has God's approval), and 3) he is God. He goes on to point out again that there are two witnesses for him, which means his testimony is still valid under the Law.
In other words, there are two senses of "valid testimony": valid in the sense of satisfying Deut. 19:15, and valid in the sense of being true. What Jesus is saying is that his testimony of himself, by itself, is not valid in the first sense but is valid in the second sense; but that God the Father also testifies for him, which makes his testimony valid in both senses.
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