وَكانوا يُصِرّونَ عَلَى الحِنثِ العَظيمِ

And they used to persist in the great sin.

EXEGESIS

Ḥinth primarily means to break a promise or deviate from a commitment, but on a more general level it is used for any sin.^{‎[1]Maqayis; Raghib; Tahqiq, under ḥ-n-th.} The general meaning is more appropriate here, but the primary meaning is used in 38:44. Since the verse says the great sin (in definite form with al), it may be deduced that it denotes a specific sin, which is best interpreted to associating partners with God (shirk), which has been identified as a great wrong (al-ẓulm al-ʿaẓīm, 31:13). And whoever ascribes partners to Allah has indeed fabricated [a lie] in great sinfulness (4:48). Another possibility is that the al denotes the genus (jins) of great sins: And they used to persist in great sins in general. Both meanings are possible and have been narrated from the early exegetes and companions.^{‎[2]Tibyan, 9/500.} Yet, perhaps the first meaning is more appropriate, especially because it is a matter of belief, and that the verses in this section of the chapter are identifying the roots and causes of misguidance and damnation. It can also be interpreted based on its specific primary meaning to untruthful solemn oaths which they used to take concerning the falsity of the resurrection: They swear by Allah with solemn oaths that Allah will not resurrect those who die (16:38).^{‎[3]Kashif, 7/225.} If so, then the next two verses would describe the content of their oath and would serve as an elaboration and clarification of this verse.

[1] Maqayis; Raghib; Tahqiq, under ḥ-n-th.
[2] Tibyan, 9/500.
[3] Kashif, 7/225.