On the use of the word testify or “witness” in the Gospel of John, see comments on 1.8.

What I say is not to be accepted as real proof is literally “My testimony is not true.” New English Bible translates this clause “that testimony does not hold good”; Phillips “what I say about myself has no value”; New American Bible “you cannot verify my testimony.” Jerusalem Bible has “my testimony would not be valid.” Moffatt translates “If I testify to myself, then my evidence is not valid.” As these translations all indicate, the point is not that Jesus is saying that his testimony concerning himself is untrue but rather that it could not be accepted as legal evidence in a court of law. The same law mentioned here is appealed to in 8.17. According to Jewish law, a man could not be convicted of a crime on the testimony of one witness (Deut 19.15). The present context, of course, is different from a situation in which a person is accused of a crime and put on trial. Here the witnesses are simply called in, as it were, to verify testimony.

As noted, it is often important to avoid a literal translation of the second part of verse 31, since it would imply that Jesus’ statements were untrue. In some languages an appropriate equivalent of verse 31 is “If I tell you about myself for my own sake, then you do not accept what I say as being true” or “… as evidence” or “If I merely speak to you on my own behalf, then no one will believe my words.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .