Albert Barnes Commentary John 14:29

"And now I have told you before it come to pass, that, when it is come to pass, ye may believe." — John 14:29 (ASV)

Before it come to pass. Before my death, resurrection, and ascension.

Ye might believe. You might be confirmed or strengthened in faith by the evidence which I gave that I came from God—the power of foretelling future events.