So how can these verses be reconciled with the mass-transmitted texts of the Qur’an and Sunnah affirming that none but the unbelievers will remain in Hell forever, and that every believer—no matter what sins short of disbelief he may have committed—must eventually leave it?

The early generations unanimously interpreted those verses in light of that established principle. The soundest explanation is that the mention of everlasting punishment for certain sins short of shirk and disbelief is a statement of causality: such sins, by their heinous nature, are a cause that would merit eternal residence in Hell were nothing to counteract it. It is, however, categorically known in Islam that faith itself blocks that permanence.

Accordingly, these texts are read in line with the well-known maxim that rulings take effect only when their causes and conditions are present and their impediments are absent. This is perfectly clear, praise be to Allah.
Furthermore, some of the cited verses themselves indicate that the “sin” in question actually means disbelief. Allah says, “and his sin encompasses him” (Al-Baqarah 2:81); sins short of disbelief do not envelop their doer entirely, for some faith remains that prevents such total encirclement.

The same holds for His words, “Whoever disobeys Allah and His Messenger and transgresses His limits, He will admit him to a Fire wherein he will dwell forever” (Al-Nisāʾ 4:14). The term “disobedience” in Scripture may refer to disbelief, to major sins, or to minor ones; once the sense of disbelief is admitted, the difficulty is removed.

Benefit: The Qur’an often speaks of a good deed being multiplied tenfold, while other verses promise even greater multiplication. How are these passages to be understood?

The answer is that the tenfold multiplication applies to every righteous act without exception, as in His statement, “Whoever brings a good deed shall have ten times the like of it” (Al-Anʿām 6:160) and similar verses found elsewhere.