John Chrysostom

He said not, ye have done it, but, ye have put a completion to it, "That as there was the readiness to will, so also [there may be] the completion also out of your ability." That this good work halt not at readiness but receive also the reward that follows upon deeds.

Homily 17 on 2 Corinthians ↗

Theophylact of Ohrid

He persuades them now not by the zeal of others, but by their own disposition, saying: you voluntarily reached the same point and not only laid the beginning of the doing, but also of the willing, that is, doing of your own good will, without any external exhortation. Therefore I advise you now to fulfill your task. So that the good work would not stop at mere zeal, but would actually be carried out. For just as disposition and free choice produce the desire, so from one's possessions comes the deed. …

Commentary on 2 Corinthians ↗

Thomas Aquinas

And because you are eager to give, therefore, now complete in deed what you had in your intention; otherwise that willingness is in vain: "Let us not love in word or speech but in deed and in truth" (1 Jn. 3:18); "And I am sure that he who began a good work in you will bring it to completion at the day of Jesus Christ" (Phil. 1:6). The reason for this is so that your readiness in desiring it, i.e., the judgment of your will, according to a Gloss is prompt, may be matched by your completing it. …

Commentary on 2 Corinthians ↗