Hebrews 3:15 (KJV)

Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways.

So I sware in my wrath, They shall not enter into my rest.)

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

But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.

For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;

For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.

But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?

And to whom sware 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.