'Unbelief' in the Bible

And he was astonished because of their unbelief.

Immediately the father of the child cried out [and] said, "I believe! Help my unbelief!"

And later, [while] they were reclining at table, he appeared to the eleven. And he reprimanded their unbelief and hardness of heart, because they did not believe those who had seen him [after he] had been raised.