Yea, they despised the pleasant land, they believed not his word:

And the head of Ephraim is Samaria, and the head of Samaria is Remaliah's son. If you will not believe, surely you shall not be established.

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

And to whom swore he that they should not enter into his rest, but to them that believed not?

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

But without faith it is impossible to please him: for he that comes to God must believe that he is, and that he is a rewarder of them that diligently seek him.

He that believes on the Son of God has the witness in himself: he that believes not God has made him a liar; because he believes not the witness that God gave of his Son.

I will therefore put you in remembrance, though you once knew this, how that the Lord, having saved the people out of the land of Egypt, afterward destroyed them that believed not.