In the book *The Kuzari*, written in Arabic in the 12th century, a rabbi (a Jewish religious leader) argues that Judaism is the true religion and that Islam and Christianity (as well as secular philosophy) are all incorrect. One of his main arguments against Islam and Christianity is that they rely on the entire nation believing the words of one prophet rather than a national revelation (like at Sinai) where everybody heard the words directly from God. 

According to the rabbi, such a story would be impossible to fake because the entire nation witnessed it. On the other hand, he says that Jesus and Mohammad could have just pretended that God spoke to them.

So why take the Prophet Mohammad at his word? How is it known that he is not making it up?