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

for if God did not spare the natural branches [because of unbelief], He will not spare you either.

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:

entered not in because of unbelief, He again, after so long a time, fixeth a certain day, saying by David, To-day; as it was said before, To day, if ye will hear his voice, harden not your hearts.