John Chrysostom
By adversaries he means not only unbelievers, but also those who do what is contrary to virtue. He teaches that our sinners will be subjected to the same fire as the adversaries.
Then, wishing to express the fierceness of this fire, he seems to fan it and says: "the zeal of fire, ready to devour the adversaries." As a wild beast, irritated, savage, and enraged, does not rest until it seizes and devours someone, so that fire also, as though inflamed with zeal, does not release the one it seizes. It devours and tears him.
Next the Apostle gives the justification for such a threat, showing that it is fitting and just. By this very fact, when its justice is shown to us, we are disposed to faith.