Theophylact of Ohrid — on 2Thess 3:4 (Commentary on 2 Thessalonians)

“After the apostle prayed for the Thessalonians and said: God is faithful and will certainly complete the work of salvation begun in you, but lest you think that everything depends only on God and therefore give yourselves over to laziness, he now requires cooperation on their part as well, as if saying: God is faithful and will undoubtedly fulfill His work, but on the condition that you also labor. Notice the wisdom of the apostle. He did not simply say: "we are confident about you," but "in the Lord," that is, we trust in the loving-kindness of God, that it will fill you with strength, in order to show them that everything depends on God. And he did not simply say: "we are confident in the Lord," but: "about you," and: "you do and will do," so that they, attributing everything to God, would not become lazy. One must entrust everything to God, but in such a way as to act oneself as well. He was not satisfied to say: "you do," but added: "you will do," showing that we must be zealous for virtue until our very last breath.”