Prophet Joseph - 12:40 Those whom you serve besides Him are but names invented by you and your fathers. Allah has not given them any power. The judgment belongs to Allah alone. He has commanded you to serve Him alone. This is the true religion. But most of the people do not know this. Those whom you serve besides Him are but names invented by you and your fathers. Allah has not given them any power. The judgment belongs to Allah alone. He has commanded you to serve Him alone. This is the true religion. But most of the people do not know this.
Questions to the Verse (18)
What is the punishment for worshipping/serving other than Allah?
All messengers conveyed the common message, "Serve Allah alone."
Majority in the Qur'an:
What did they serve besides Allah?
How did they serve others besides Allah?
Why did they serve others besides Allah?
Some reasons why we should serve Allah alone:
To make religion belong to Allah alone / to serve Allah alone:
Related:
The true religion:
A reminder to those who take angels, prophets, religious leaders, etc. as gods besides Allah and serve them, hoping that they will help them on the Day of Judgment:
Satan made the people under his influence invent gods, laws and prohibitions and attributed them to Allah and thus misguided them from the right path:
There is no proof/revelation/book from Allah that supports their taking gods and thus associating partners:

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