This phrase should probably be attached to v. 7, as further describing the coming of Christ. The context and the general teaching of Scripture seem to support this. At the great advent day the Lord will be revealed in His own glory, with the glory of His Father, and of the holy angels (see Luke 9:26). Such glory appears to mortal eyes as fire. Thus Moses described the glory of God (Ex. 3:2), and thus, Ezekiel (Eze. 1:27), Daniel (Dan. 7:9, 10) and John the revelator (Rev. 1:14, 15).

 Gr. ekdikēsis (see on Rom. 12:19). The phrase may be translated, “inflicting punishment.”

 Paul conceives those whom the Lord punishes as belonging to two classes, those who know not God and those who obey not the gospel. Some interpret these classes as the Gentiles and the Jews respectively (cf. Jer. 10:25; Rom. 10:16), but it seems better to think of them as two general classes of people. The first are those who have had opportunity to know God but have spurned the privilege (see Ps. 19:1-3; Rom. 1:18-21). The second class are those who know the gospel message but have refused to obey it. Their reasons for rejecting the gospel are clearly shown by the Lord Himself to be their love of sin (see John 3:17-20).