I was raised in the Bible Belt. I went to Baptist churches more than any other, and at some services and revivals, I was told by the leaders and other congregants that Catholics, Mormons and/or Methodists weren't really Christians. And my Catholic friends have told me that they were told at various times that all non-Catholics weren't really Christian.
In Egypt, I was told by a colleague, a Sunni Muslim, that Shi'ites aren't really Muslims. Bosnian friends who consider themselves Muslim were told by people visiting from Saudi that, in fact, Bosnians were real Muslims.
After a doctor that provided abortion services is shot in the USA, people will say, "Oh, he wasn't really a Christian." And after a bombing by someone claiming to be Muslim, other Muslims will exclaim, "Oh, he wasn't really a Muslim. That's not true Islam." The Taliban will kill Muslims in Afghanistan and say it was justified, because they weren't really Muslim. Malala Yousafzai is quoted as saying, "The Taliban think we are not Muslims, but we are. We believe in God more than they do..."
I've heard Buddhist debate who is really a Buddhist, Jews debate who is really a Jew, and I'm guessing every other religion has members debating this.
Who decides what is religiously authentic? Who gets to decide who gets called what?
As an Atheist, I'm frequently confused by all these I-am-this-but-that-one-is-not claims. If someone says he or she is a practitioner of this or that religion, then that's how I refer to that person - and as a result, I've had some indignant responses from people who also identify as a part of that religion, who don't think that person's version is the "real" version.
To be honest, I'm sick of it. I'm sick of the back and forth. Do people who engage in this rhetoric not realize how incredibly silly they sound? Why do YOU get to decide who gets called what, but not the person you criticize? That person believes that a/the god is on his or her side just as much as you do, and has just as much dogma and holy book/holy person quotes to back up his or her point of view as you do. In a debate, you both would end up being guilty of cherry-picking phrases and rules to back up your claim.
I wish I could side with Malala Yousafzai in saying her version of Islam is the real one, and the Taliban's isn't. I wish I could side with the United Church of Christ over, say, the Southern Baptist Convention, in terms of which version of Christian practice and teaching is the true one. But I can't. As an Atheist, I just can't. But I can most definitely say which I prefer, which I would like to have the most adherents, if said people simply MUST believe in a magical, invisible super friend.
So, yes, I do roll my eyes when someone of a faith reacts to an act of violence by someone else of faith by saying, "Well, he/she isn't really Christian / Muslim / Jewish / Hindu / Buddhist / Whatever." Because that person who committed the violence is saying the exact same thing about said speaker.
There are universal horrors we, as humans, should all be condemning, not because of a religion, but because of our humanity: murder, sex trafficking, child molestation, sexual violence, oppression, ethnic "cleansing", and on and on. And there are universal virtues and conditions we, as humans, should all be celebrating and embracing: kindness, understanding, compassion, inspiration, freedom, equality, equal access, safety, security, and on and on. I judge you based on your practices with regard to these universal virtues - and you don't need a god to have them, nor a label.