For Christians, calling yourself a Christian while not believing that God has always existed as the triune Father, Son, and Holy Spirit is as inconceivable.
This is not simply a conservative evangelical Christian view. Methodists have said "the LDS Church is not a part of the historic, apostolic tradition of the Christian faith." Even Roman Catholics (hardly conservative Protestants) don't recognize LDS baptism.
The problem is that, in America, everybody's an expert: If you say you are xyz, you are xyz. Even though you most definitely, unequivocally, are not xyz.
Russell Kirk once said that Christianity wasn't a failure, it's just that it has never really been tried. Quite the condemnation, that, on Paul, Augustine and Luther among others, when you think about it. Or on Thomas Aquinas.
I'll go him one better, though, since fools rush in where angels fear to tread: Jesus had no disciples in his lifetime, and he's never had any since. He just hasn't been around to correct the record which states otherwise.
At most one might venture to say that Jesus has had imitators who took themselves almost as seriously as he took himself.
But apart from that opinionated air, it is probably more useful for the issue at hand to accept at face value the early observation that "Christian" was in truth an epithet applied by outsiders. It was not originally a term of self-description:
"And in Antioch the disciples were for the first time called Christians" (Acts 11:26).
Jews in particular understood believers in Jesus like Paul to be members of a sect of Judaism, a cult if you will, which was not officially recognized, in a way similar to how Christians today do not recognize Mormonism, which borrows from Christianity quite freely and builds something new on it.
Interestingly enough, the self-designation which Paul mentions in referring to this fact is follower of "The Way":
"But this I admit to you, that according to the Way, which they call a sect, I worship the God of our fathers, believing everything laid down by the law or written in the prophets" (Acts 24:14).
That self-description goes back directly to the teaching of Jesus:
"Enter ye in at the strait gate: for wide [is] the gate, and broad [is] the way, that leadeth to destruction, and many there be which go in thereat: Because strait [is] the gate, and narrow [is] the way, which leadeth unto life, and few there be that find it" (Matthew 7:13f.).
For Paul, those belonging to "the few" became an increasingly larger number beyond just the lost sheep of the house of Israel:
"Go not into the way of the Gentiles, and into [any] city of the Samaritans enter ye not: But go rather to the lost sheep of the house of Israel" (Matthew 10:5f).
But there Paul did go, and the rest, as they say, is history. Which I think goes a little way toward explaining religious innovation in our own time, Mormon innovation included.