And the disciples were called Christians first in Antioch.
Obviously the name Christian was first applied to people who were followers of Christ. It was more than likely applied in a derogatory manner at the very beginning but quickly became a name to die for.
Since Paul's letters indicate that Churches and individual Christians were already deviating from truth at the very beginning, we know that there were probably people who called themselves Christian, but were not. They were followers of another Jesus. Paul said of them that they should be accursed. At that same passage we also received instruction that if they were teachers of another Gospel the same result. We can accurately infer that unless it is what Paul preached, it is not truth. Therefore unless a Christian is like the Christian Paul, then they must not be Christian. Here at least is a good start to end the frustration. It may seem to border on heresy to tell a Christian that he is not a Christian. At what juncture does a self proclaimed Christian no longer fit the name? What form of accepted doctrine would disqualify right to the name? What type of behaviour would remove access to the name?
I cannot end this article without stating some definites. Jesus said, "Ye must be born again to enter the Kingdom of God!" It does not matter what you call yourself. God knows who are true Christians and who are not. It is up to us to search the only source of criteria, the Bible, come to a conclusion and stick with it come what may. The world will go on with all kinds of people claiming to be of Christ. The Word of God is the only authority for us to follow. We can only follow it's Christ.