'Unbelief' in the Bible

Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.

So we see that they could not enter in because of unbelief.

Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief:

Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.

By faith [with confidence in God and His word] Noah, being warned by God about events not yet seen, in reverence prepared an ark for the salvation of his family. By this [act of obedience] he condemned the world and became an heir of the righteousness which comes by faith.