Well reading your post I would ask, what is mainstream, it is certainly not mainstream that all Christian think Muslims are going to hell or vicky verky...
Maybe you're forgetting about the US, especially the evangelicals? I could't find much on hell but here's some stats on what US Christians think about who goes to heaven.

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Source]
So 50% of evangelicals and 31% of all Christians in the US think you have to believe in
their version of Christianity.
...I personally think Islam is a beautiful religion, from my reading it is based mostly on the word "Charity" in fact this is their Holy month, a time to reflect, how lucky they are and that others are not so fortunate, and if you read my favourite part of the New Testament Corinthians book 1 Chapter 13 KJV, it is the word "Charity" not love that is used.
And we all know the KJV is the original.

More seriously, you can't deny that there is the Taliban and so-called 'Islamic State' who would "accept God". Probably most Muslims don't think they're right, but they nevertheless exist.
Jehovah witnesses not mainstream....
But there's quite a lot of them and they do "accept God".
...but with all the big religions one overarching sense, I call it God, Muslims call it Allah, Hindu's call it Brahmin, Buddhists call it Nirvana, Tao call it the way, the path.
But all these different ideas contradict each other. There's no getting around that. The belief that there is only one god-thingy behind it all—that at least most people have got seriously wrong—is just
another god-thingy, and one that appears to have serious communication problems.
A sense that there is more to us than just evolved monkey/ape brains, and as the theory of evolution grows I think it will prove just that.
Ah, good old blind faith.
To end, if you would like to read more, this thing I call a sense, a feeling, a gut instinct...
The thing is people have a 'gut instinct' about all sorts of superstitions, not only versions of God. Astrology, the paranormal, spirits, ghosts, aliens visiting the earth, magic, homeopathy, crystal healing, the list is endless. We're naturally superstitious. We see agency and purpose where there is none. It makes perfect sense from an evolutionary point of view.
...Karen Armstrong, but it comes with a Gonnagle warning, may fry the most ardent of atheist minds.
Does she have any objective reason (solid objective evidence and/or sound reasoning) to take the idea of God at all seriously? If so, do share. If not, why would I care what she has to say about God or religion?