Just a quick message about the "You have to choose Jesus, or Muhammad, or reject them both, but you can't accept them both" idea.. I disagree, unless one refuses to accept the idea that Jesus' true message hasn't been faithfully retained in the New Testament. To a Christian, who believes the entire NT is the "Word of God", and who believes Jesus is God incarnate, of course, that person can not accept Muhammad's message. Then again, one could say, "You have to choose God, or Jesus, or reject them both, but you can't accept them both", if you're a Jew. And that is how some Jews think today, such as one book's title "You take Jesus, I'll take God". Since the Hebrew Bible more than once states that the Jews are to keep the law "forever", or however that's translated, many Jews have a difficult time accepting the idea that they don't need to keep the Law, so to them, the NT is preaching a different God. The Christian could respond mentioning that Jeremiah passage which speaks of a "new covenant", but if you read it, it says this applies to the house of Israel and the house of Judah (looks like it might be referring to the twelve tribes to me, and were the twelve tribes even around in Jesus' day?), it says "my law" will be placed in their hearts. The Law, not another law, or another religious code. Again, I'm speaking from a possible Jewish viewpoint.
And to a Muslim, if he believes the Qur'an, and accepts the idea that Muhammad was the final prophet, then he can not believe in Guru Nanak's message, or Baha'ullah's, or Joseph Smith's, etc.
BTW, if one reads the alleged "debate" between Celsus and Origen, from what I recall, Celsus says that the mystery religions' adherents threatened eternal damnation to the Christians. So this idea was not unique to Christianity, apparently. What if one of the mystery religions was actually the true religion?