'Unbelief' in the Bible

What then? If some did not believe, will their unbelief cancel God’s faithfulness?

He did not waver in unbelief at God’s promise but was strengthened in his faith and gave glory to God,

True enough; they were broken off by unbelief, but you stand by faith. Do not be arrogant, but be afraid.

And even they, if they do not remain in unbelief, will be grafted in, because God has the power to graft them in again.