John Chrysostom

"But ye, brethren, are not in darkness, that that day should overtake you as a thief." Here he speaks of a life that is dark and impure. For it is just as corrupt and wicked men do all things as in the night, escaping the notice of all, and inclosing themselves in darkness. For tell me, does not the adulterer watch for the evening, and the thief for the night? Does not the violator of the tombs carry on all his trade in the night? What then? Does it not overtake them as a thief? …

Homily on 1 Thessalonians 9 ↗

Theophylact of Ohrid

That is, you do not lead a dark and impure life. So then, what? Will the end not come upon them unexpectedly, even if they are not impure? It will come unexpectedly, but it will bring them no sorrow. For a thief can do no harm to those who are awake and have light, even if he manages to enter their house — on the contrary, those who are in darkness and sleeping he robs completely and departs.

Commentary on 1 Thessalonians ↗

Isaac of Nineveh

Now if the good God sees that a man's heart has not inclined to any of these things as David said, indicating the same, "You have tested my heart, You have visited it in the night, You have tried me by fire, and unrighteousness was not found in me," then God will help him and deliver him. Why does he say "in the night" and not "in the day"? Because the enemy's deceptions are a night, as Paul also said, "We are not children of the night but children of the day," since the Son of God is the Day, but Satan is night.

The First Syriac Epistle of Saint Makarios of Egypt 7

Augustine of Hippo

On the verse: "This is the day which the Lord has made." What we have sung to our Lord let us put into practice with his help. To be sure, every day has been made by the Lord, but with good reason has it been said of a particular day, "This is the day which the Lord has made." We read that when he created heaven and earth, "God said, 'Let there be light,' and there was light, and God called the light Day, and the darkness Night." But there is another day, well established and definitely to be commended by us, concerning which the …

Sermons 230 ↗

Thomas Aquinas

Then Paul says, but you are not in darkness, brethren, and explains what he had mentioned in regard to the good people; and he does this by making two points: first, he excludes the good people from the company of the evil people and secondly, he provides a reason for this (5:5). And so he remarks, you are not in darkness, for you have been enlightened by Christ concerning that day; this is not an unexpected event for you. "He who follows me will not walk in darkness, but will have the light of life" (Jn. 8:12).

Commentary on 1 Thessalonians ↗