It is a fearful thing to fall into the hands of the living God. David, when the option was given him, preferred falling into the hand of the LORD to falling into the hand of man (2 Samuel 24:14), trusting in the greatness of his mercies.
But the case contemplated here is that of its being "too late to cry for mercy, when it is the time of justice." Fearful (the writer would say) is the thought of being exposed, without possibility of escape or of atonement, to the wrath of the Eternal Righteousness.
The inspired author of this Epistle had evidently an awful sense of the Divine wrath against sin, and of man's liability to it without atonement. He felt deeply the contradiction between humanity as it is and its ideal of perfection; and hence the wrath attributed to God in Holy Writ would appear to him as inseparable from a just conception of Divine holiness.
For the more ardent the love in the human heart of moral good, by so much the keener is the indignation against moral evil, and the sense of the righteousness of retribution. The existence of such evil at all in the good God's universe is indeed a mystery; but, as long as it is there, we cannot but conceive the face of the holy God as set utterly against it; and so any revelation to us of the Divine nature would be imperfect did it not include the idea which is humanly expressed by such terms as "zeal," "jealousy," "wrath," "vengeance."
Hence came the long-felt need of some atonement, to reconcile sinful man to the eternal holiness. This need was expressed of old by the institution of sacrifice, which, however—as is so clearly perceived in this Epistle—could never itself be really efficacious in the spiritual sphere of things.
In the atonement of Christ (if rightly apprehended) is found at last a true satisfaction of this spiritual need. But, man's concurrence being still required, the idea of Divine wrath remains notwithstanding, as operative against such as, in deliberate perversity of free-will, after full knowledge, refuse to be thus reconciled.
Hence the awful anticipations of future judgment on some, contained in this Epistle. The nature and duration of the doom to come, on such as remain subject to it, are in these passages left in obscurity.
They speak only of φοβερά τις ἐκδοχὴ, an undefined expectation of something terrible. It may be observed, however, that, whatever be the force of other Scriptures in which the fire of that day is described as eternal and unquenchable, here at least the figure of a zeal of fire to devour the adversaries seems in itself to suggest rather utter destruction than perpetual pain.