John Gill 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 came to pass
This is a strong proof of his true and proper deity, for none but the omniscient God can tell of things before they come to pass; this is peculiar to him, and distinguishes him from the gods of the Gentiles; see (Isaiah 41:22Isaiah 41:23) (45:21) (Isaiah 46:9Isaiah 46:10) ;

that when it is come to pass, you might believe :
that is, that when he was removed from them, and gone to his Father, they might then believe that he was truly God, the Son of God, the promised Messiah; and that he was then in glory, and at the right hand of God.