But there was another group in Western history which lay claim to the title of Christians, a group which also departed crucially from the historic shape of Christian orthodoxy: the medieval Cathars.
Cathars making a convert away from the Catholic Franciscan friars.
There are fascinating similarities between the two groups as well as deep differences. The Cathars (Greek for "pure ones"), or Albigensians (from their central stronghold, the southern French town of Albi) also denied the Trinity*, judged the historical Christian Church to be irredeemably corrupt, had a separate sacramental system and priesthood, a restricted canon as well as supplementary scriptures. They also rejected veneration of the cross. Cosmic dualists, they viewed Jesus as a pure created spirit sent in the guise of human flesh by the Good Father God, who was at war with the co-eternal Evil Satanic God who had created the world of matter and imprisoned their souls in bodies on the earth.
[The best book on the subject is Yuri Stoyanov's (2000) The Other God: Dualist Religions from Antiquity to the Cathar Heresy.
Incidentally, it was confrontation with this group which sparked St. Dominic to found his community of preachers. His first foundation was a monastery of nuns, all women whom he had converted from Catharism to Catholicism.]
What really separates Cathars from Mormons is religious dualism, a tradition shared by Zoroastrians, Gnostics, and Manicheans. While perfect Cathars dualists abstained from marriage (and considered escape from this world through starvation --the endura--the ultimate religious achievement), it is precisely through marriage for eternity and childbearing that the very non-dualist, practical and world-affirming Mormons achieve perfection.
Although their motives for rejecting orthodoxy are very different, the somewhat similar forms of distance from it that both Mormons and Cathars assert makes it impossible to include them within the Christian religion. To do so would empty the category of little meaning beyond "expressed religious veneration of Jesus Christ, regardless of doctrinal content."
That's hard to sell to a Five.
*The Mormons accept a trinity, but Heavenly Father, Jesus Christ and Holy Spirit are three separate gods united only in a harmony of purpose. Hence, they are polytheists, not Trinitarians.