“From now on I am telling you before it comes to pass, so that when does occur, you may believe that I am He.” (John 13:19)

This is true not only regarding Jesus’s impending death and resurrection, but of ALL Christian prophecy. WHY did God give these words? “so that when it does occur, you may believe.” Prophecy demonstrates that something Divine is indeed operative here on earth:  

— When Psalm 22, written 1000 years before the birth of Christ, opens with the very words Jesus cried out on the cross: “My God, My God, why have You forsaken Me?”

— When Isaiah 53 so clearly foretells the life and ministry of Jesus as a Suffering Servant that even Jewish readers, unaware of the source of the text, admit this is a  description of Jesus Christ …

— When even Jesus’ enemies knew in advance that He had predicted that He would rise from the dead on the 3rd day, but they could do nothing to stop it … it causes people’s ears to perk up, and take notice.

Because God told us these things in advance, and they did occur, it leads us to believe. Jesus indicates here that this is the purpose of Biblical prophecy.