Since MORMONism is 'restored' Christianity; you'll have to ask them to PROVE the very same thing!
(Making them a very SMALL cult; right?)
On the other hand, you've got Mormon Christianity, whose doctrines about "who is Christ" and "what is his relationship to God" were established by human councils, during the nineteenth century.
Now frankly I think that Joe Smith was making it all up, but I don't see any reason why the other is any more valid, or even any more Biblical.
Prior to the councils convened by the early Catholic Church, there were many competing flavors of Christianity, and there is nothing wrong with that.