But a certain fearful looking for of judgment - The word “certain” here does not mean “fixed, sure, inevitable,” as our translation would seem to imply. The Greek is the same as “a (τις tis) fearful expectation,” etc. So it is rendered by Tyndale. The idea is, that if there was voluntary apostasy after having embraced the Christian religion, there could be nothing but an expectation of the judgment to come. There could be no other hope but that through the gospel, and as this would have been renounced, it would follow that the soul must perish. The “fearful apprehension” or expectation here does not refer so much to what would be in the mind itself, or what would be experienced, as to what must follow. It might be that the person referred to would have no realizing sense of all this, and still his situation be that of one who had nothing to expect but the terrors of the judgment to come.
And fiery indignation - Fire is often used in the Scriptures as an emblem of fierce punishment. The idea is, that the person referred to could expect nothing but the wrath of God.
Which shall devour the adversaries - All who become the adversaries or enemies of the Lord. Fire is often said to devour, or consume, and the meaning here is, that those who should thus become the enemies of the Lord must perish.