John Chrysostom

"Let us fear lest a promise being left us of entering into His rest, any of you should seem to come short of it; for to us was the Gospel preached as well as unto them when it is said, To-day if ye hear His voice" (for "To-day" is "at every time"). Then he adds "but the word of hearing did not profit them, as they were not mixed by faith with them that heard." How did it not profit? Then wishing to alarm them, he shows the same thing by what he says.

Homily on Hebrews 6 ↗

Theophylact of Ohrid

Here is a return to the passage where it says: "today, if you hear His voice" (Heb. 3:7), and then in order: "let us fear," lest the same thing happen to us as well. The word "therefore" is placed to resume the discourse that was interrupted by a lengthy repetition. That is, lest they be completely deprived of the possibility of entering that promised rest. Softening and tempering his speech, he did not say "lest he be late," but "lest he be found to have been late."

Commentary on Hebrews ↗

Oecumenius

"Let us therefore fear." In the saying, he says, "Today, if you will hear His voice, do not harden your hearts as in the rebellion (Heb. 3:15), let us therefore fear." And the "therefore" has been added. For the sake of being lengthy, so that the construction may not seem incoherent, the "therefore" has been placed. Let us fear, he says, lest any of you should seem to come short of entering into his rest. For the promise of entering into his rest still remains for those who wish. And this is the kingdom of heaven. …

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

Having cited the authority and explained it, the Apostle now argues from it. In regard to this he does two things: first, he urges them to be anxious to enter; secondly, he advises them to hasten to enter (v. 11). In regard to the first he does two things: first, he inspires them with fear; secondly, he shows that anxiety should press them (v. 2). He says, therefore: It has been stated that He was offended by those who would not believe, so that He swore that they will not enter into His rest. …

Commentary on Hebrews ↗