Why will they be saying, "Peace and safety"?

I Thessalonians 5:2 For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.

  3 For when they shall say, Peace and safety; then sudden destruction cometh upon them, as travail upon a woman with child; and they shall not escape.

  4 But ye, brethren, are not in darkness, that that day should overtake you as a thief.

Apparently, the "they"/"them" are those still in darkness when the day of the Lord comes.

They will not have gotten saved, they will still be lost.

Those still in darkness either do not believe that the day of the Lord is coming, or are blinded to its nearness, or both.

Why would they be saying, "Peace and safety"?

Will that be something they will say in response to something?

Or is it something that they will always be saying?

Or will it be like the false prophets in Judah, before the captivity?

Apparently it will be more than them simply being wrong, but rather, sudden destruction comes upon them.