Tafsir of Al-Infitar 82:9

"Nay" (Kalla) is a deterrent against being deluded by the generosity of Allah Almighty and using it as a pretext for disbelief and acts of disobedience, despite the fact that it [His generosity] should necessitate gratitude and obedience.

His saying—Exalted is He—"Nay! But you deny the Recompense" (bal tukaththibuna bi-ad-din) is a shift (idrab) from an implied clause to which the discourse naturally leads. It is as if it were said after the deterrence by way of objection: "You do not desist from that; rather, you grow bold toward something even greater, for you deny the Recompense and the Resurrection entirely," or [you deny] the religion of Islam, both of which are among its [the Quran's] rulings. Thus, you believe neither in the questioning nor the answering, neither in the reward nor the punishment. In this, there is an escalation from the lighter [sin] to the more severe.

According to al-Raghib, bal here serves to rectify the second [statement] and invalidate the first. It is as if it were said: "There is nothing here that necessitates their delusion; rather, their denial is what drove them to what they have committed."

It has been said that the implication of the discourse is: "You do not remain steadfast upon what My favors upon you and My guidance to you necessitate; rather, you deny, etc."

It has also been said that "Nay" (Kalla) is a deterrent against what this clause indicates—their denial of the Resurrection and the Gathering. Then bal is a shift from an implied meaning; it is as if it were said: "The matter is not as you claim regarding the denial of the Resurrection and the Raising," then it was said: "You are not enlightened by this clarification; rather, you deny, etc."

Kharijah, on the authority of Nafi‘, assimilated the 'lam' of Kalla into the 'ra' of rakabaka [in the previous verse], as did Abu ‘Amr in his Idgham al-Kabir. Al-Hasan, Abu Ja‘far, Shaybah, and Abu Bishr read yukaththibuna (they deny) using the third-person 'ya'.