Theophylact of Ohrid

It would seem that this speech is inconsistent, but in reality it is not: it speaks of the fact that no one can say that David is speaking of the Sabbath rest. For how can he speak of it, when it had long since passed and the world had received its ordering from the very beginning? It is clear that David speaks of some other rest. This rest pertains to the future, since obviously someone must enter it after this. …

Commentary on Hebrews ↗

Oecumenius

Indeed, he says, "the third rest is the kingdom of heaven. And there again, "If they shall enter into my rest." Behold, therefore, two works, the other indeed of which David said: "If they shall enter into my rest."

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

Then he cites the authority of David, which has already been explained. But when he says, since, then, it remains for some to enter it, he argues from the premises. In regard to this he does two things: first he accepts the sense of the second authority; secondly, he argues from the first (v. 9). In regard to the first he does two things: first, he extracts two things from the second authority; secondly, he shows that those two things are understood in it (v. 8).

Commentary on Hebrews ↗