I'm not sure what you mean by Allah belongs to EVERYBODY. I think you may be using the word 'belongs' in a confusing fashion. Allah is not OWNED by anybody. If you mean (which is what I think you do) he is the God of everybody i.e. the sole creator of all mankind and that no human has an exclusive right over Allah then I understand your point. Sorry, its proberly just the wording.

The Jews, Chrisitans and the Hindus all actually believe in the EXISTENCE of the one, exact and same God i.e. Allah - the one true God who has no partners, rivals etc.

However, Islamically this belief in his existence ALONE does not make one a Muslim as one must believe that He (Allah) is the ONLY one to be worshipped and the crux of that is to worship Allah AS HE PRESCRIBED. After all we humans who are prone to errors could not devise error-free ways to worship God properly. Worship would become nonsensical over time.

So the worship God has prescribed for mankind MUST be adhered to strictly and hence the fundamental belief in the Qur'an, The Last Messenger Muhammad (peace be upon him) etc. Without these beliefs and followings one is not a Muslim.