So did Hilaire Belloc, but the fact is you can't be a heretic wrt a group you've never belonged to. Am I a Jewish heretic? Mohammed and his followers had some remote and fleeting exposure to Christianity, but they weren't claiming to be acting as Christians. They were proposing a final prophecy, but as alien proselytisers. If you appear in Arabia waving a book dictated to you by an angel or some such, and proclaiming a new religion that replaces Islam, does that make you an Islamic heretic? No, it makes you a nutjob.
Only if your mother was Jewish. One of the differences between Judaism on the one hand, and Christianity and Islam on the other, is that most Jews are Jews by birth.
If you appear in Arabia waving a book dictated to you by an angel or some such, and proclaiming a new religion that replaces Islam, does that make you an Islamic heretic?
That makes you Baha'u'llah.
Only if your mother was Jewish.
Come to think of it, you would, in that case, technically be an "apostate".