Let me clarify my point a bit...what I am getting at is how do we tell if a group of believers is Christian or not? Individual beleivers is one thing; only God can be their judge. But can't we and shouldn't we try to determine if sects, cults, churches, etc are Christian? How can we do this?
I believe we can do this by holding to two non-negotiables: the Trinity and the efficacy of Christ's death to bring about salvation.
Here's an example. Is Benny Hinn's ministry Christian or not? (Note that I asked if his ministry was Christian, not whether or not he was because that's God's job not mine). Well, Benny Hinn breaks the first of my non-negotiables -- he doesn't believe in the Trinity: he believes that each person of the Trinity is a Trinity, which places him outside of the Christian tradition. Thus, Benny Hinn's ministry is not Christian.
Another example, T.D. Jakes ministry. Jakes does not believe in the Trinity, therefore his ministry is not Christian. Jakes believes that there is not a Trinity, instead in the unity of God...part of the Oneness movement.
Other examples could be Jehovah's Witnesses and Mormons who don't believe properly about Jesus' nature as expressed in the Bible and Christian tradition, thus their churches are not Christian.
Any group of people who teach that someone can go to heaven in any way other than through faith in Jesus, who's death on the cross opened the way for us to have eternal access to God, is not Christian. This also would excise any group who taught Jesus plus anything gets one to heave, like the opponents of Paul in Galatians who taught that Jesus plus Law overservance was required.
It should be noted that there is much room left for disagreement without pulling out the "you're not a Christian!" card. We can disagree about exactly how Jesus' death is efficacious, about the observances of the church, about modes of baptism, etc, etc. But the two things that we must hold to are the Trinity and the efficacy of Jesus' death for salvation.