Not All Faith is Saving Faith
Not everyone who believes has a believing faith. It is one thing to believe in and another to believe on. It also matters what you are believing in and on. Jesus knows the heart! Therefore, He does not entrust Himself to every one. He only saves those who genuinely believe in Him and not in His signs alone.
Borchert sums it up nicely in the context of this Gospel:
When John wrote this Gospel, he knew that Jesus performed many signs and that people said they believed. John also knew that Jesus died and that while he had no intention of abandoning the believers (14:18), they could not avoid abandoning him. For John, then, there was good reason for Jesus not to believe people’s believing. Thus, when we read the stories of John, we must not treat them simply as stories from the past. They are also in fact living portraits of humanity in every era. Accordingly, we need to understand that the living Jesus does not believe everyone’s believing because he knows what is in them. Those words ought to stand as a warning to everyone (Borchert, p. 168).