Charles Ellicott 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)

And now I have told you before it come to pass.—Compare John 13:19. Here, again, He tells them the event before its accomplishment, so that it may serve to strengthen their faith.

Two interpretations of this verse are possible:

1. That He told them of the coming of the Advocate to teach all truth and bring all things to their remembrance, so that in its fulfillment, they may, with increased faith, believe in Him.
2. That He told them of His going to the Father, so that when the hour of His departure came, they may believe that He had gone to the Father.

On the whole, and especially considering the close parallel with John 13:19, the first interpretation seems the more probable meaning.