'Unbelief' in the Bible

See, brethren, lest there shall be in any of you an evil heart of unbelief in the falling away from the living God,

and we see that they were not able to enter in because of unbelief.

since then, it remaineth for certain to enter into it, and those who did first hear good news entered not in because of unbelief --

May we be diligent, then, to enter into that rest, that no one in the same example of the unbelief may fall,