The Koran’s sura 4:89 declares: “They would have you disbelieve as they themselves have disbelieved, so that you may all be alike. Do not befriend them until they have fled their homes for the cause of Allah. If they desert you, seize them and put them to death wherever you find them. Look for neither friends nor helpers among them.” (Emphasis added.) Koranic passages are taken by believers to be the words of God Himself.
To the extent there is interpretive gloss on these scriptures, most authoritative are the Hadiths — the traditions and admonitions of Mohammed and his companions. According to Abdullah Ibn Abbas, Mohammed’s cousin and among the most influential educators in both Sunni and Shiite traditions, the prophet’s instructions in this regard were quite clear: “Kill him who changes his religion.” Indeed, as Jihad Watch’s Robert Spencer recounted in The Myth of Islamic Tolerance, the only real argument in Islam about apostasy pertains to the nature of the penalty — beheading or some different method. There is no credible dispute about whether it is a high crime or whether death is an appropriate sentence.