'Unbelief' in the Bible

And he was amazed because of their unbelief. Then he went around among the villages and taught.

Immediately the father of the boy cried out and said, "I believe; help my unbelief!"

Then he appeared to the eleven themselves, while they were eating, and he rebuked them for their unbelief and hardness of heart, because they did not believe those who had seen him resurrected.