But you, brothers, aren't in darkness, that the day should overtake you like a thief.
that that day should overtake you as a thief: One who lives in a constant state of readiness will not be surprised by the coming of the Lord regardless of when it occurs. There may be a suggestion that the thief is so engrossed in his nefarious activities that he is oblivious to the approaching dawn that will expose his misdeeds. Those who fail to maintain a state of vigilance and readiness will similarly be overtaken by the coming of the day of the Lord.