Her sin appears to be trifling to some. It says in verse 26 that she "looked back." It may seem little, but it reveals a great deal about her character. She directly disobeyed the clear command of God's messenger just given a few verses before that. We read it. I Samuel 15:22-23 says that "to obey is better than sacrifice . . and . . . rebellion is as the sin of witchcraft." She rebelled.
Ominous - very ominous words. In terms of faith, what John said in that last paragraph puts this chapter into a very different perspective. Everyone hearing God's word is confronted with a choice: believe it and obey it, or take the chance of dying.
I wonder if you noticed in these verses, as compared with the rest of the story, that suddenly there is a change in the pronouns from plural to singular. Notice verse 17, "So it came to pass, when they had brought them outside, that he said." One of the messengers speaks. In verse 18 then Lot addresses him and it says in my Bible, "lords." Keil & Delitzsch Commentary says, "No, that word "Lord" is singular." Lord. Adonai - the name of God. Is it the name of Melchizedek? Was Melchizedek the one that was there to destroy the city? If it was not, why did he call him Lord, and why in verses 21 and 22 does He takes the authority to himself to destroy the city? "I cannot do anything until you arrive there. And then it says, "The Lord rained down . ." Is the Lord the same one who was the "I" of the previous verse?