He had no doubts at first, but slowly they emerged. He was struck, he says, by the fact that the first coins bearing Muhammad’s name did not appear until the late 7th century — six decades after the religion did. He traded ideas with some scholars in Saarbrücken who in recent years have been pushing the idea of Muhammad’s nonexistence. They claim that “Muhammad” wasn’t the name of a person but a title, and that Islam began as a Christian heresy. Prof. Kalisch didn’t buy all of this. Contributing last year to a book on Islam, he weighed the odds...