Sure, after we add Scientology.
No, they are not. They do not believe in Holy Trinity dogma, which makes them non christians. Or, at vey least, should make them non-christians in game terms, since chiristians of all denomination do not consider them as such.
Not accepting the Nicean Creed is not grounds for being not Christian. The Council of Nicea was not an elected body with real power to decide matters of faith. The Christ revering Gnostics were as Christian after the Council as they were before. Believing in Christ as a prophet is the minimally sufficient grounds for one religion to be considered a Christian. Mormons are Christian.