I think that the Trinitarian hype has overshadowed this idea of who or who is not a Christian, when really it should be based on whoever believes in Jesus Christ, regardless of his identity, personage, and role in salvation. This would include Jehovah's Witnesses, Christadelphians, those from the Latter-day Saint movement, and Liberal Christians.
For me, the idea of one who is a Christian is summarised in John 17:3,
"And this is life eternal, that they should [1] know thee the only true God, and [2] him whom thou didst send, even Jesus Christ."