en_tn/1th/05/04.md

you, brothers

Here "brothers" means fellow Christians.

are not in darkness

Paul speaks of evil and ignorance about God as if they were darkness. Alternate translation: "you are not unknowing, like people who live in the dark" (See: rc://en/ta/man/jit/figs-metaphor)

so that the day would overtake you like a thief

The Lord will come on a day when believers will not expect him to come, but that day will not "overtake" them—God will not destroy them on that day—because they will be ready for Jesus to return. (See: rc://en/ta/man/jit/figs-simile)