With respect to religion, the fallacy is well used, often even overused. Religious apologists will repeatedly try to use the No True Scotsman argument to distance themselves from more extreme or fundamentalist groups, but this does not prevent such extremists actually being religious - they themselves would certainly argue otherwise. Moderate Muslim leaders, for example, are well known for declaring Islamic extremists as "not true Muslims" as Islam is a "religion of peace." Similarly, moderate Christians, such as those in Europe, are sometimes aghast when viewing their fundamentalist counterparts in the US, immediately declaring them "not true Christians," even though they believe in the same God and get their belief system from the same book. Many of these statements stating that the extremists are not true believers are often used as a reaction against Guilt by Association.

The No True Scotsman fallacy can also run the other way when it comes to extremism. Extremists will make a religious statement and when someone points out that there are many believers who don't believe the extremist's viewpoint, the moderates are deemed to be not true believers (ie: Christians who support gay marriage are not "real Christians" or Muslims who support women's rights are not "real Muslims". Modern pagans do it all the time, perhaps even more than other religions, due to the fact that there is no agreed-on orthodoxy for the whole group, with some well-established practices in one setting being considered unpalatable in others. Silver Ravenwolf, one of the best selling "leaders" of neopagans, has done this with multiple ancient, well-established practices.

It's a tricky business, as being a member of a religious group, to the minds of those involved, encompasses adhering to a certain standard of behavior. For example, charity can certainly be called an essentially Christian ethic, considering the emphasis that Jesus placed on it. The man himself would most definitely disavow the greedy and "What's mine is mine" mindset of many right-wingers who call themselves Christians. However, strictly speaking, a Christian is defined as "one who professes belief in the teachings of Jesus Christ"; there's no rule saying they have to do it right./div]