2 articles on the Islamic sources, texts and religion that undergird what ISIS / ISIL / IS does

1.  A very thorough article at the Atlantic magazine.
http://www.dennyburk.com/is-isis-terror-driven-by-religion-the-high-cost-of-ignoring-the-obvious/

The Full article:
http://www.theatlantic.com/features/archive/2015/02/what-isis-really-wants/384980/

2.  Dr. White's analysis:
http://www.aomin.org/aoblog/index.php/2015/02/16/breaking-the-cross-killing-the-swine-truly-thinking-about-isis-and-the-murder-of-21-copts/

Addendum:  "do not be surprised" at persecution for the sake of Christ
http://www.thegospelcoalition.org/blogs/justintaylor/2015/02/16/tom-schreiner-a-biblical-meditation-on-the-isis-execution-of-21-christians/

Addendum 2:
See my plea for Balance in seeking to understand the conflict within Islam today.  Christians, don't let fear keep you from reaching out to the other Muslims who are not Jihadist types. 

Understanding Dhimmi ذمّی in Islam

The Concept of Dhimmi   ذمّی   in Islam.