The one who saw this happen has spoken of it is best interpreted in the light of 21.24: He is the disciple who spoke of these things. The question which disciple is meant seems answered by the fact that, apart from Jesus’ mother, only the disciple he loved (19.36) is said to have been present at the crucifixion. As far as translation is concerned, a third person reference should be maintained. There is no exegetical basis for the Living Bible’s “I saw all this myself and have given an accurate report so that you also can believe.”

Has spoken of it is literally “has testified” or “has witnessed.” What he said translates the noun “testimony” or “witness.” (See comments at 1.7.)

Good News Translation‘s restructuring makes clear that so that you also may believe expresses the purpose of the eyewitness’ testimony (The one who saw this happen has spoken of it) and not simply of his telling the truth. In the Greek sentence structure he speaks the truth immediately precedes the purpose clause.

It is better not to supply an object for believe, as Die Bibel im heutigen Deutsch does (“therefore you also may believe it”), since here the verb means not simply accepting the truth of what the eyewitness has said but coming to faith in a theological sense. Compare Goodspeed “to lead you also to believe.”

There is a textual problem connected with you … may believe. Some manuscripts have the Greek verb in the present tense, implying the meaning “that you may keep on believing.” Other manuscripts have the aorist tense, which, strictly interpreted, means “that you may come to believe.” If the present tense is accepted, the intimation is that the Gospel was written to believers, to strengthen their faith. The aorist tense would imply that it was written for non-Christians, to call them to believe. No final decision is possible, and the UBS Committee on the Greek text has left the choice open. (See at 21.3, where the same problem exists.)

He knows that he speaks the truth may be rendered “he knows that what he has said is true” or “he knows that what he has said actually happened.”

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 .