41 Lot also, the brother of Abraham, was just and hospitable in Sodom, and was found worthy to escape the conflagration which prefigured the future judgment; for he was free from all participation in the corruption of the people of Sodom. He was a type of the body of Christ, which in the person of all the saints both groans now among the ungodly and wicked, to whose evil deeds it does not consent, and will at the end of the world be rescued from their society, when they are doomed to the punishment of eternal fire.
Lot's wife was the type of a different class of men—of those, namely, who, when called by the grace of God, look back, instead of, like Paul, forgetting the things that are behind, and looking forward to the things that are before. The Lord Himself says: "No man that puts his hand to the plough, and looks back, is fit for the kingdom of Heaven." Nor did He omit to mention the case of Lot's wife; for she, for our warning, was turned into a pillar of salt, that being thus seasoned we might not trifle thoughtlessly with this danger, but be on our guard against it.
So, when the Lord was admonishing every one to get rid of the things that are behind by the most strenuous endeavor to reach the things that are before, He said, "Remember Lot's wife." And, in addition to these, there is still a third type in Lot, when his daughters lay with him. For here Lot seems to prefigure the future law; for those who spring from the law, and are placed under the law, by misunderstanding it, stupefy it, as it were, and bring forth the works of unbelief by an unlawful use of the law. "The law is good" says the apostle, "if a man use it lawfully."
Source: Reply to Faustus the Manichaean (New Advent)