The claim
Eternal punishment in Hell, as described in 78:21-26, is disproportionate and therefore unjust — finite sins committed in a short life cannot justify an infinite penalty.
The kernel of concern
The concern is morally serious and worth taking on its own terms: a just penalty should fit the crime, and on its face an unending punishment for the deeds of a limited lifespan looks like an extreme mismatch. The passage itself raises the bar by insisting in 78:26 that the requital is exactly 'fitting' (wifāq), so the question of proportion is fair to ask.
What the verse actually says
The passage does not describe punishment for ordinary mistakes or for a brief lapse. The dwellers are al-ṭāghīn — 'those who transgress' — whom the surah has already characterised as those who persistently denied the resurrection and the message (78:27-28: 'they did not expect an account, and they denied Our signs with emphatic denial'). And 78:26 frames the entire outcome as jazāʾan wifāqan: a recompense that precisely corresponds to the deeds, which the commentators (al-Saʿdī, Ibn Kathīr, al-Qurṭubī) read as a statement of exact justice, not excess.
The honest position
On the Islamic view this is genuinely just rather than merely asserted to be. The gravity of a wrong is measured not only by its duration but by what is wronged: persistent, knowing rejection of the Creator is treated as the gravest offence, since 'there is no sin greater than associating partners with God' (al-Qurṭubī). Permanence of state is also tied to the unrepentant orientation of the person, not merely to a tally of acts. Notably, the same tradition holds that monotheist sinners are eventually removed from the Fire (al-Qurṭubī), so perpetuity attaches specifically to settled, defiant rejection — not to weakness or sin as such.
Strongest counter-arguments
A steel-manned defence: (1) Many ethical systems already accept that the wrongness of an act is not a function of how long it took — a moment's act can warrant a lifelong consequence. (2) The 'short life' framing assumes punishment tracks the time spent sinning rather than the disposition of the agent; if the rejection is permanent and unrepentant, a permanent state is not obviously mismatched. (3) The Qur'an repeatedly stresses God's mercy precedes His wrath and that none is wronged 'even by the weight of an atom' (4:40), so the wifāq in 78:26 is a claim about exact justice, with mercy operating widely up to the point of settled, willful denial.