The early church had the same problem with groups with heretical doctrines wanting to be called Christian and thinking theirs was the only true church. Today, we call them Gnostics and they were determined by the Councils not to be considered Christian.
The Gnostic heresies are precisely why the early creeds were developed and the canon solidified. They are both measuring sticks of orthodox (correct) doctrine versus heterodox doctrine.
And many who insist that you can be outside orthodox Christian doctrine and still be a ‘Christian’, forget that there is a term called “pseudo-Christian” which is what most people actually mean when they say Mormons are not Christian, that they are ‘pseudo-Christian’, simply based upon their heterodox doctrines and practices.
[Gnostics] were determined by the Councils not to be considered Christian. The Gnostic heresies are precisely why the early creeds were developed and the canon solidified. They are both measuring sticks of orthodox (correct) doctrine versus heterodox doctrine.
So, Mormons are not Christians because they do not follow the creeds and canon defined by a committee of men some 300 years after Christ, but instead follow the creeds and canon defined by a committee of men some 1800 years after Christ.
I do agree that we need to define this differently, though. I think I will start using the phrase Trinitarian-Christian. Mormons are definitely not Trinitarian-Christians. Of course, many would argue that the early apostles and even Christ Himself were not Trinitarian-Christians, either.