Theophylact of Ohrid

"This we will do" – what? – that is, to go on to perfection, if God also wills it. He said this not as though God did not command it, but as he usually spoke, that if it is pleasing to God, then this is precisely what I also want to do. And at the same time He teaches us here that we should make everything dependent on His will and that even in unquestionably good deeds we should not trust our own judgment and our own strength. The Apostle Jude also clearly persuades us of this same thing.

Commentary on Hebrews ↗

Oecumenius

"And this we will do." Which? To be carried to perfection. "if God permits." The phrase "if God permits" is not to indicate that God does not always permit (for God always permits the good and perfect), but as is customary for us to say, If God wills, we will do this. At the same time, it also teaches us from this that everything depends on that will, and we should not rely confidently or arrogantly on our own judgment and power, even in matters generally acknowledged. This is explicitly encouraged by the apostle James.

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

286. – Then (v. 3) he shows how difficult it is to carry out his intention. For it is difficult both in itself and in relation to his hearers. Hence he does three things: first, he suggests that in this he especially needs divine help; secondly, he mentions their weakness (v. 4); thirdly, he proposes a simile (v. 7). 287. – He says, therefore: Let us go on to things more perfect, and this shall we do, if God permit. …

Commentary on Hebrews ↗