'Unbelief' in the Bible

yet, looking unto the promise of God, he wavered not through unbelief, but waxed strong through faith, giving glory to God,

Well; by their unbelief they were broken off, and thou standest by thy faith. Be not highminded, but fear:

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