John 14:29 KJV
And now I have told you before it come to pass, that, when it is come to pass, ye might believe.
Here is an Old Testament principle practiced by the Lord. The people of the Old Testament were to verify the truthfulness of a prophet by this one evidence; if what he said came to pass he was if God. Here Jesus tells His disciples that his purpose in telling them what would happen to Him served to give them reason and right (authority) to believe.