John Chrysostom

"Seeing therefore it remaineth" (he says) "that some must enter therein, and they to whom it was first preached entered not in because of unbelief: again he limiteth a certain day, saying in David, To-day, after so long a time; as it has been said before." But what is it that he means? "Seeing then" (he means) that "some must" certainly "enter in," and "they did not enter in." And that an entrance is proclaimed, and that "some must enter in," let us hear from what this is clear. …

Homily on Hebrews 6 ↗

Oecumenius

For if some must enter, and those who entered have not entered into the rest of the promised land, it again defines a third rest, so that, as he says, some may enter into it.

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

Therefore, he extracts two things: one is well known, namely, that the ancestral fathers did not enter; the other is that during the time of David there remained another rest to be given. For although the promised rest was to be obtained in the promised land, yet by the fact David so long afterward says, Today, if you hear his voice, harden not your hearts, he shows that another rest remains. Otherwise, he would have made no mention of the rest, saying, Today if you hear his voice, harden not your hearts. …

Commentary on Hebrews ↗