Lest these believing Thessalonians should be terrified in their minds by this discourse, he adds this by way of comfort to them, that they shall not be surprised as others; though they did not know the particular time of Christ's coming, yet it would not find them unprepared for it as the world would be; and the reason he gives is, because they are not in darkness. Darkness is to be taken metaphorically; and so in Scripture it is taken either for sin, ignorance, or misery. The two former are here meant, especially ignorance. These Thessalonians were brought into the light of the gospel; they had the knowledge of Christ, and the way of salvation by him; particularly they knew of his coming, and the manner and ends of his coming, which the infidel world did not; and though Christ's coming would be to others as a thief in the night, yet not to them.
Related Commentaries of 1 Thessalonians 5:4
1 Thessalonians 5:4
4 But ye, brethren, are not in darkness, that that day should overtake you as a thief.