Exempted from automatic execution, it appears, are Christians who do not resist their new government. Baghdadi permits them to live, as long as they pay a special tax, known as the jizya, and acknowledge their subjugation. The Koranic authority for this practice is not in dispute.

There is obvious ego at play here – http://www.theatlantic.com/magazine/archive/2015/03/what-isis-really-wants/384980/?utm_source=SFFB