Theodore of Mopsuestia

Paul seems to say here that the activity of grace has grown, for he does not say that “we give thanks” but that “we are bound to give thanks.” In the same way we deserve to have a complaint lodged against us if we do not render to someone what is justly owed. This is the force of “as is fitting,” where the point is that there has been an increase in the work of grace, such that what has happened in the Thessalonians is rightly referred to God as its source.

Paul shows here just how seriousness this punishment is by his use of “eternal.” Indeed the punishment of those wicked who have died is completed in a reality that transcends time itself and is forever.