John Chrysostom

"Being confident of this very thing, that He which began a good work in you will perfect it until the day of Jesus Christ." See how he also teaches them to be unassuming. For since he had witnessed a great thing of them, that they may not feel as men are apt to do, he presently teaches them to refer both the past and the future to Christ. How? By saying, not, "Being confident that as ye began ye will also finish," but what? …

Homily on Philippians 1 ↗

Theophylact of Ohrid

I give thanks, he says, and "I rejoice," in the hope that God, "who began a good work in you," will Himself also "bring it to completion," because from the past I draw conclusions about the future as well. See how he teaches them to think humbly of themselves, when he ascribed everything to God and not to them alone; however, by this he in no way deprived them of honor, since he said: "who began in you," that is, according to your desire. …

Commentary on Philippians ↗

Thomas Aquinas

And being confident of this very thing, I take joy in you because, he who began a good work in you will bring it to completion: "Cursed is the man who trusts in man" (Jer. 17:5), and in (17:7): "Blessed is the man who trusts in the Lord, whose trust is the Lord." And this by God's power; hence he says, he who began a good work in you will bring it to completion: "Apart from me you can do nothing" (Jn. 15:5). This is against the Pelagians, who say that the principle of every good work is from ourselves, but its completion is from God. …

Commentary on Philippians ↗