John uses the word “belief” and its derivatives frequently through his Gospel. One minute everyone believes, the next minute the same believers want to kill Jesus. The disciples say they believe several times and yet Jesus seems to say otherwise.
A phrase that Jesus uses a couple of times with the 12 disciples is “that ye may believe.” He looks forward to a time when they would believe Him.
What does He mean by this? Repeatedly it says the disciples believed, why does He put their faith in the future?
Faith is an ongoing thing. The just shall live by faith. Unbelievers may believe for a time, but then they fall away when they read something they don’t like.
The disciples saw and heard many things that brought about faith in them. But Jesus knew that the battle was not over. Would they believe when He was crucified? When He was buried? When He ascended to heaven?
He gave them words to rely on in those tough moments, a ground for faith in the midst of confusion. Eleven of the twelve continued believing. They believed til the end. This is always the proof of true, saving faith: it continues.