what has millennia of the "no true christian!!!" arguing accomplished?

I mean, not much, but rejecting the teachings of Jesus is still literally heresy. If we are to have a useful definition of the word "Christian," it kinda has to include believing in the word of Christ.

If someone tells everyone he knows he is a man, he is a man even if he doesn't seem like a man sometimes. Same with Christians. It is really annoying when they don't seem to follow many Christian teachings but they are still Christian. IMO anyway.