Many Christians in particular and theists in general harbor the notion that those that are true practitioners of their faith are incapable of performing particular actions. For example, many Christians maintain that true Christians do not engage in premarital or extramarital sex, smoke cigarettes, do drugs, dink alcoholic beverages, etc.
Most Muslims claim that true Muslims do not and cannot engage in terrorist threats or actions. This idea often takes a bigoted twist. For example, after the terrorist attacks on 9/11, many Muslims claimed that the attackers were not true Muslims. Some went further and proclaimed that they must have been atheists, because no one that truly believes in God could be capable of such actions.
However, this mindset is not limited to theists. Many humanists claim that true humanists cannot be sexist, racist, homophobic, etc. However, this notion that someone that truly embraces a worldview is somehow incapable of performing immoral actions is terribly flawed.