What does Genesis 19:15 mean?

During the prior night, two angels disguised as men had arrived in Sodom. Lot offered to protect them in his house, knowing they would not be safe in the streets overnight. A mob arrived at Lot's home, demanding these strangers be sent out so they could be raped. When this same crowd attacked Lot for interfering, the strangers pull Lot back into the house and supernaturally blind the attackers. They warn Lot to go and call anyone else he wishes to save from death to leave Sodom immediately. Lot does so, but nobody listens.

Here, a long night in Sodom finally comes to an end. It will be the last night before God's judgment rains down on the city. The angels make clear to Lot and his family that the time has come for the four of them to go to avoid being swept away in the punishment of the city. The language used here is the same that Abraham used when asking the Lord if He would sweep the righteous away with the wicked. The Lord had assured Abraham that He would not destroy the city if He found ten righteous people. He did not—the city of Sodom is completely and utterly depraved.

Still, the Lord will be merciful to "righteous Lot" (2 Peter 2:7) and his family, likely because of God's covenant promises to bless Abraham and all those with him.