Adam Clarke's Commentary on the Bible

Verse 7 (Jude 1:7)

Even as Sodom and Gomorrha - What their sin and punishment were may be seen in Genesis 19, and the notes there. This is the third example to illustrate what is laid down Judges 1:4 .

Are set forth for an example - Both of what God will do to such transgressors, and of the position laid down in Judges 1:4 , viz., that God has in the most open and positive manner declared that such and such sinners shall meet with the punishment due to their crimes.

Suffering the vengeance of eternal fire - Subjected to such a punishment as an endless fire can inflict. Some apply this to the utter subversion of these cities, so that by the action of that fire which descended from heaven they were totally and eternally destroyed; for as to their being rebuilt, that is impossible, seeing the very ground on which they stood is burned up, and the whole plain is now the immense lake Asphaltites. See the notes on Genesis 19 (note).

The first sense applies to the inhabitants of those wicked cities; the second, to the cities themselves: in either case the word πυρ αιωνιον signifies an eternally destructive fire; it has no end in the punishment of the wicked Sodomites, etc.; it has no end in the destruction of the cities; they were totally burnt up, and never were and never can be rebuilt. In either of these senses the word αιωνιος , eternal, has its grammatical and proper meaning.

- Adam Clarke's Commentary on the Bible