The Catholic Church has a very specific definition of who is a Christian and who is not. Mormons such as Kimball_Kinnison and Oneness Pentecostals such as Jabba-wocky do not match it. However, I think that in terms of friendly, reasonable discussion, we should call anyone who claims to be a Christian a Christian. If I were a sociologist I'd consider Unitarian Universalism, Mormonism, and Eastern Orthodoxy all to be variants of the Christian religion. In general, obnoxiously telling someone they aren't X when they openly profess to be X is bad for dialogue and just plain rude.