In نَارُ اللَّهِ, the rāʾ of nāru carries ḍammah immediately before the Divine Name. Therefore its lām is pronounced heavily.
Contrast بِسْمِ اللَّهِ, in which a preceding kasrah makes the lām light. The difference comes from the preceding vowel, not a different word.
In the same verse, الْمُوقَدَةُ has a clear article lām because mīm is a moon letter.
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Nāru llāh: a heavy lām in the Divine Name
Al-Māʿūn and Al-Humazah: Applied Assimilation, Concealment, Madd and Hāʾ Rules
In نَارُ اللَّهِ, the rāʾ of nāru carries ḍammah immediately before the Divine Name. Therefore its lām is pronounced heavily.
Contrast بِسْمِ اللَّهِ, in which a preceding kasrah makes the lām light. The difference comes from the preceding vowel, not a different word.
In the same verse, الْمُوقَدَةُ has a clear article lām because mīm is a moon letter.