وَالْكُفْرُ نِهَايَةٌ فِي الْجِنَايَةِ
Unbelief is the extreme degree of offense.
This argument contends that an offense of such gravity cannot admit pardon. The response distinguishes the enormity of wrongdoing from a demonstration that pardon is logically impossible. Declaring a prohibited act permissible differs from forgiving the penalty of an act already committed.
For example, legalizing a wrong would change the applicable rule, whereas pardoning a penalty would not make the underlying conduct lawful. That distinction matters when testing the argument.