One thing that I have wondered about Muhammad was whether he himself genuinely believed in Islam. I don't think he was lying about his experience in the cave (most likely a hallucination), but on the other hand we have incidents in which Mo makes up quran verses for his own convenience, such as this one:
Quran 33:53 O believers, enter not the houses of the Prophet, except leave is given you for a meal, without watching for its hour. But when you are invited, then enter; and when you have had the meal, disperse, neither lingering for idle talk; that is hurtful to the Prophet, and he is ashamed before you; but God is not ashamed before the truth. And when you ask his wives for any object, ask them from behind a curtain; that is cleaner for your hearts and theirs. It is not for you to hurt God's Messenger, neither to marry his wives after him, ever; surely that would be, in God's sight, a monstrous thing.
The above verse is quite obviously made up for Mo's ends rather than ''God's'' which would imply that if he was willing to falsify verses, his belief was probably not sincere.