So, as per them, Mormons are not real Christians, SDAs are not real Christians, ...; why, if this goes on long enough, there won't be any real Christians left.
I suspect a lot of them are Baptists (and I apologize to any Baptists on the board in advance who don't fit this description). It's a narrow mindset I find a lot in Baptist circles.
(As for me—and standard disclaimer, I was raised as a Methodist—I think that, in general, those denominations are Christian, since the same fundamental belief in salvation through Christ is the same in all of them. That isn't to say that everything they believe about it is accurate, but much of it, at least, isn't harmful, simply a matter of personal taste.)