How many times in discourse with Christians have you heard this, or something similar? It is the quintessential example of the “No True Scotsman” fallacy.
I've always found entertaining self appointed Christian declarations of who amongst their fellow Jesus worshippers are or aren't "True Christians". I have seen fundamentalists call Catholics "Satan worshippers" and “Idolaters”; the Pope "The Anti-Christ"; and declare their brethren in less fanatical protestant sects "unsaved". I've witnessed so called loving Christians call Mormons “heathens” at worst , and "cultists" at best [I don’t disagree, after all aren’t all religions cults?].
Certainly, most of the 2,800+/- Christian sects distain and deny the flavor of Christianity the Jehovah’s Witnesses’ practice for their rejection of the non-Biblical concept of Trinity. In fact the level of disgust “mainstream Christians” have for JWs is almost as epic as their distain for Jews.
Finally, there is the not so occasional religious homicidal maniac, like the church going anthrax scientist who recently committed suicide, or the child killing mother in the throes of religious fanatic delusion … all dismissed as “not True Christians". How very convenient, and self serving.
And should one Christian’s actions cause embarrassment or place fellow Christians in a bad light…oh, well… that’s the “not True Christian” kiss of death. This in spite of the doctrine that says one is not condemned by bad acts, nor saved by good acts or good deeds, simply by acceptance of Jesus as The Savior.
So what drives this need to disown their own out of hand? Two things:
1. To be a member of one of those “other cults”, or to have alternate positions on social issues, threatens the mainstream Christian flock members’ certainty of the validity of their own sect and positions.
2. Christians who embarrass other faithful by their unsavory / illegal acts even when done at the direction of Jesus, casts all Christians in a bad light and undercuts the credibility of the “true” Christians (as if Christianity had any credibility to begin with).
Hey, if you tell me you don't believe in god/s I promise I won't question your "True atheism"… even if you are a pervert, anti-choice, send anthrax letters, and /or vote Republican.