Theophylact of Ohrid

He shows that they had good works and only needed to be perfected in them. This is what he also prays for on their behalf. See that we ourselves must first begin, and then ask for perfection from God. He who has been perfected in every good deed does the will of God, not he who does good only halfway. Consequently, when we do good, it is God who accomplishes it in us through Jesus Christ, that is, through Him who serves as Mediator and Perfecter. …

Commentary on Hebrews ↗

Oecumenius

"equip you in every good work." That is, to fulfill, to perfect. It shows, therefore, that they have begun the good works; they needed to be perfected in them; Paul who also prays for them. But see, that we must begin first, and then ask from God for the end. "That you do His will." For he who is equipped and filled for every good work, he has the strength to do the will of God. And he says every work, work and faith. …

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

Then he adds his petition when he says, May he equip [fit] you with everything good. For the human will, since it is the inclination of reason, is the principle of human acts, as heaviness is the principle of downward movement of heavy bodies; hence it is related to the acts of human reason as a natural inclination to natural acts. But a natural thing is said to be fit for that to which it has an inclination. So, too, man, when he has the will to do good, is said to be fit for it. …

Commentary on Hebrews ↗