This verse is similar to 13.19. The adverb now is emphatic in the Greek sentence structure.

Believe may be understood in an absolute sense (New English Bible “may have faith”), but in the present context it seems better to take it with the meaning “believe me” (Die Bibel im heutigen Deutsch), that is, “believe that what I said is true.”

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 .