'Unbelief' in the Bible

{What is the result} if some refused to believe? Their unbelief will not nullify the faithfulness of God, [will it]?

And he did not waver in unbelief at the promise of God, but was strengthened in faith, giving glory to God

Well said! They were broken off because of unbelief, but you stand firm because of faith. Do not think arrogant [thoughts], but be afraid.

And those also, if they do not persist in unbelief, will be grafted in, because God is able to graft them in again.