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Cf. 3:191 . The above statement appears in the Qur'an in several formulations; see, in particular, note [11] on 10:5 . In the present instance it connects with the mention of the Day of Reckoning in the preceding verse, thus leading organically from a specific aspect of David's story to a moral teaching of wider import.
I.e., a deliberate rejection of the belief that the universe - and, in particular, human life - is imbued with meaning and purpose leads unavoidably - though sometimes imperceptibly - to a rejection of all moral imperatives, to spiritual blindness and, hence, to suffering in the life to come.
Cf. iii. 191. Unbelief is the subjective negation of a belief in Order, Beauty, Purpose, and Eternal Life. Unbelief is to Faith as Chaos is to Cosmos, as the Fire of Misery is to the Garden of Bliss.