You can’t really be claiming that Muhammed is to Islam what Luther is to Christianity! Muhammed invented Islam. Sure, he claimed to be a prophet in the true line of Jesus, Moses and Abraham, but that’s as specious as Joseph Smith’s claims. He started the religion and wrote it’s holy books. Nothing like Luther.
And what gives you the idea that only one source claims Muhammed’s impeccability in Islamic teaching? (I have certain problems with Spencer myself) Granted it’s only been a small sample (including the rare poster here), but I’ve yet to meet a muslim who believes Muhammed ever committed a sin (or any of the real prophets, actually).
I’ll grant you that some of what Muhammed actually did in his life (as portrayed in the Koran) probably embarrasses and confuses the vast majority of muslims who would love to find a way to explain it away and try very hard to do so (because they are decent people). But you’re kidding yourself if you believe that Islamic teaching sees Muhammed the way Christians see Luther (or even the saints). I’m aware of no Lutheran in history ever to claim the sinlessness of Luther. The very idea is an oxymoron!
But once again, we’re just flailing at each other here. I’ll have to live with not changing your mind!
P.S. Islam is both a culture and a religion. There’s no “Aha!” involved in criticizing Islamic culture versus Islamic faith. It’s like the head and tail of a coin: can’t be separated.