In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
All Commentaries on 2 Thessalonians 1:8 Go To 2 Thessalonians 1
Thietland of Einsiedeln
AD 1000
by a flame of fire
Perhaps it is about that which is read in the Apocalypse of John, "and fire came down from heaven and consumed them." (Rev 20;9)
who are not obedient to the Gospel
And this even refers to evil Catholics who do not obey the Gospel of our Lord though they say they believe, but they do not live according to what they believe.