But the brutal targeting of Christians highlights a deeper theological issue among Muslims: interpreting what qualifies as shirk, or the belief of equating any being with God. Various translations define shirk as “paganism,” “idolatry” and “polytheism,” and the people who practice shirk are mushrikun. It’s a historical reference to the pre-Islamic, polytheistic religious practices in 7th-century Mecca, before the prophet Muhammad started preaching belief in one God—tawhid in Arabic. The young men in the Islamic State propaganda videos flash a gang sign of one finger pointed skyward to represent monotheism, something ordinary Muslims also do.

Mushrikun is a slur word like “Witch!” during the Salem, Massachusetts, witch trials of 17th-century America. A Muslim detractor first called me a mushrikun in 2005 after I organized a mixed-gender Friday prayer led by a woman imam, or imama. It’s code for “enemy of Islam.”

The Quran (4:48) spells out that shirk is a “major sin,” stating: “And he who practices shirk has certainly fabricated a tremendous sin.” In one hadith, shirk is worse than murder. While most Muslims cast Christians and Jews as ahl-e-kitab, or “people of the book,” several elements of Christian practice—the deification of Christ, with belief in the “Father, the Son and the Holy Ghost,” the veneration of saints and the iconography of Mary and Jesus—make Christians a target for extremists like those in the Islamic State as mushrikun.