The someone else referred to in this verse is the Father, and in translation it may be best to make this explicit, since the next person mentioned in the passage is John, at the beginning of the next verse.

In this context one may translate testifies as “says what he knows about me” or “indicates who I am.”

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 .