From a recent survey at the Public Religion Research Institute:

Americans employ a double standard when evaluating violence committed by self-identified Christians and Muslims.

More than 8-in-10 (83 percent) Americans say that self-proclaimed Christians who commit acts of violence in the name of Christianity are not really Christians. In contrast, less than half (48 percent) of Americans say that self-proclaimed Muslims who commit acts of violence in the name of Islam are not really Muslims.