Long before the first Hebrew temple, before the birth of Christ or the mission of Muhammad, there lived in Persia a prophet to whom we owe the ideas of a single god, the cosmic struggle between good and evil, and the Apocalypse. His name was Zarathustra, and his teachings eventually held sway from the Indus to the Nile and spread as far as Britain. Following Zarathustra's elusive trail back through time and across the Islamic, Christian, and Jewish...