“Now I have told you this
Before it occurs.
When it does occur,
You may believe.”
καὶ νῦν εἴρηκα ὑμῖν πρὶν γενέσθαι, ἵνα ὅταν γένηται πιστεύσητε.
John uniquely indicated that Jesus said that he had told them this (καὶ νῦν εἴρηκα ὑμῖν) before it occurred (πρὶν γενέσθαι), so that when it did occur (ἵνα ὅταν γένηται), they would believe (πιστεύσητε). Jesus was telling them this now so that when it actually happened, they would believe him. He wanted to show them that he knew what was going to happen to him. Do you believe in Jesus and his teaching?