السَّلَامُ عَلَيْكُمْ
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لا- no
إله- Illah
إلا- except
الله- Allah
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The question is what is an إله/ “illah”

The false meaning of “illah” – one who is
-the creator
-the sustainer
-the provider
-the controller of the universe
-sends down the rain
-the provider
-answers the dua

The issue is that the Quraysh believed all these things about Allah. Yet they were disbelievers who were fought against. So why did the Quraysh have such a problem with لا إله إلا الله. It’s because إله means something else.

The true meaning of “illah” –
one that is worshipped.

The Quraysh had no issues with believing Allah was the Creator, the Sustainer, etc. But they had an issue with worshipping Allah alone.

Allah Azza w Jaal says:

{ وَسۡـَٔلۡ مَنۡ أَرۡسَلۡنَا مِن قَبۡلِكَ مِن رُّسُلِنَآ أَجَعَلۡنَا مِن دُونِ ٱلرَّحۡمَٰنِ ءَالِهَةٗ يُعۡبَدُونَ }
[Surah Az-Zukhruf: 45]

And ask (O Muhammad صلى الله عليه و سلم) those of Our Messengers whom We sent before you: “Did We ever appoint âlihâh (gods) to be worshipped besides the Most Gracious (Allâh)?”
— Muhammad Taqi-ud-Din al-Hilali & Muhammad Muhsin Khan(43:45)

-Notice how Allah mentions ءَالِهَةًۭ(plural of إله) next to يُعْبَدُونَ (to be worshiped). This shows the “illah” is something to be worshipped.

Allah knows best.