John Calvin Commentary 1 Thessalonians 5:3

"When they are saying, Peace and safety, then sudden destruction cometh upon them, as travail upon a woman with child; and they shall in no wise escape." — 1 Thessalonians 5:3 (ASV)

For when they shall say. Here we have an explanation of the comparison, the day of the Lord will be like a thief in the night. Why is this so? Because it will come suddenly to unbelievers, when unexpected, so that it will take them by surprise, as though they were asleep.

But from where does that sleep come? Certainly, it comes from a deep contempt of God. The prophets frequently reprove the wicked on account of this careless indifference, and indeed, they await in a spirit of carelessness not merely that final judgment, but also those judgments that occur daily. Though the Lord threatens destruction, they do not hesitate to promise themselves peace and every kind of prosperity.

And the reason they fall into this destructive idleness is this: because they do not immediately see accomplished those things which the Lord declares will take place, they consider anything to be a fable that does not immediately appear before their eyes.

For this reason, to avenge this carelessness, which is full of stubbornness, the Lord comes all of a sudden. Contrary to everyone's expectation, He hurls the wicked from the height of their prosperity.

He sometimes provides signs of this kind of sudden coming, but the principal one will be when Christ will come down to judge the world. As He Himself testifies (Matthew 24:37), He compares that time to the age of Noah, since everyone will be given over to excess, as if in the deepest rest.

As the pains of child-bearing. Here we have a very fitting comparison, since there is no affliction that seizes more suddenly, or presses more keenly and violently from its very first onset. Besides this, a pregnant woman carries the cause of her pain in her womb without feeling it, until she is seized by it in the midst of feasting and laughter, or in the middle of sleep.