'Unbelief' in the Bible

Yet, looking to the promise of God, he didn't waver through unbelief, but grew strong through faith, giving glory to God,

True; by their unbelief they were broken off, and you stand by your faith. Don't be conceited, but fear;

They also, if they don't continue in their unbelief, will be grafted in, for God is able to graft them in again.