If they shall enter. That is, they shall not enter. See See Barnes "Heb 3:11".

The object of quoting this here seems to be two-fold.

(1.) To show that even in this Psalm God spoke of his rest, and said that they should not enter into it; and

(2) it is connected with Heb 4:6, and is designed to show that it was implied, that a rest yet remained. "That which deserves to be called the divine rest is spoken of in the Scriptures; and as they did not enter into it, it follows that it must be in reserve for some others, and that the promise must still remain."