'Unbelief' in the Bible

What then? If some did not believe, does their unbelief nullify the faithfulness of God?

He did not waver in unbelief about the promise of God but was strengthened in faith, giving glory to God.

Granted! They were broken off because of their unbelief, but you stand by faith. Do not be arrogant, but fear!

And even they -- if they do not continue in their unbelief -- will be grafted in, for God is able to graft them in again.