'Unbelief' in the Bible

Nor did he in unbelief stagger at God's promise, but became mighty in faith, giving glory to God,

This is true; yet it was their unbelief that cut them off, and you only stand through your faith.

Moreover, if they turn from their unbelief, they too will be grafted in. For God is powerful enough to graft them in again;

For God has locked up all in the prison of unbelief, that upon all alike He may have mercy.