“I have told you now …” It was right that the disciples should be warned about this frequently, for it was a secret far beyond all human grasp. He declares that he foretells what will happen, so that when it happens they may “believe.” It was a useful confirmation of their faith when they recalled Christ’s predictions and saw happening before their eyes what they had previously heard from his mouth. Yet it seems to be a sort of concession, as if Christ had said, “Because you are not yet able to comprehend such a deep mystery, I bear with you till the event has happened, which will be an interpreter to explain this teaching.” Although at the time he seemed to be speaking to the deaf, afterwards it became apparent that his words had not been scattered in vain or (so to speak) in the air, but that they were seed sown in the ground. Now as Christ here speaks about his Word and what will happen, so his death and resurrection and ascension to heaven combined with his teaching to produce faith in us.
Calvin, J. (1994). John (Jn 14:29). Wheaton, IL: Crossway Books.