Riiiiight. So who decides that? You? God?
What about Mormons? Jehovah's Witnesses? or are they not real Christians because they don't believe in the real teachings? Paulian or Gnostic?
It's something Christians cannot accept, but there is no determination for a true Christian. If that god does exist, it's literally between an individual and God.
On this planet, it's self-identified.