John Calvin Commentary 1 Thessalonians 5:4

"But ye, brethren, are not in darkness, that that day should overtake you as a thief:" — 1 Thessalonians 5:4 (ASV)

But you, brethren. He now admonishes them about the duty of believers: to look forward in hope to that day, even though it may be distant. And this is what is intended in the metaphor of day and light.

The coming of Christ will take by surprise those who are carelessly giving way to indulgence because, being enveloped in darkness, they see nothing; for no darkness is more dense than ignorance of God. We, on the other hand, on whom Christ has shone by the faith of his gospel, differ greatly from them, for that saying of Isaiah is truly fulfilled in us:

while darkness covers the earth, the Lord arises upon us, and his glory is seen in us (Isaiah 60:2).

He admonishes us, therefore, that it would be an unseemly thing for us to be caught by Christ asleep, as it were, or seeing nothing, while the full blaze of light is shining upon us.

He calls them children of light, in accordance with the Hebrew idiom, meaning furnished with light; and also children of the day, meaning those who enjoy the light of day. This he again confirms when he says that we are not of the night nor of darkness, because the Lord has rescued us from it.

For it is as though he had said that we have not been enlightened by the Lord with a view to our walking in darkness.