The Argument for Islam is very Simple:

[Crunchy]

Or maybe that an actual leader arose from that community.

If someone presented evidence for that hypothesis I would certainly consider it.  The simple fact is that the bullshit written about "mohammed" dates from the same 80-100 year period after his supposed death that we see with 'jesus.'  And jesus is phony shit, too.

https://en.wikipedia.org/wiki/History_of_Saudi_Arabia


The history of Saudi Arabia shows that "someone" conquered the place around 600AD and that "someone" spread islam around that time. Unless you think it was god, it had to be somebody. Almost all historians agree that it was a guy named Mohammed. This is not at all the same as Jesus as there is contemporaneous evidence from non Muslim sources of Mohammad's existence. 

Having said this, it is irrelevant (even about Jesus). It seems to me that too many atheists embrace the "non existence" of these religious figures with glee because it's sticking it to the religious. 
There is no primary evidence for the existence of Homer, Pythagoras or Socrates, but that doesn't matter when evaluating their works and no one bothers to mention it about them.

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Proof for Islam?

I believe in Islam.  It exists.  There is conclusive proof for this everywhere.

What I don't believe is Islam's message.

Exactly. 
When I say Mohammed was a  murderer, rapist and pedophile, it is an accurate representation of what is said in the Quaran, and Hadith. I oppose islam based on what the text say, not what any individual chooses to redact from his own mind.