'Unbelief' in the Bible

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

yet, with respect to the promise of God, he did not waver in unbelief but grew strong in faith, giving glory to God,

Quite right, they were broken off for their unbelief, but you stand by your faith. Do not be conceited, but fear;

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