Christians too claim that their God is the only God. So how do you know that what you believe is true?
Allah is no god - any god who insists that he alone be worshipped and threatens those who worship others with eternal hellfire is no better than a mafia boss.
Muhammad was not the messenger of any god but just an ordinary human who was suffering from delusions and thought that he was receiving messages from god. We can find many such Muhammads in any mental asylum. The only difference is that Muhammad was able to gather around him ignorant people who believed his absurd claims and spread it passionately with the sword.
