IndigoChild5559
Loving God and my neighbor as myself.
For me as a Jew, the answer is simply that a Christian is anyone who believes that Jesus is the messiah and God.We know that a true Scotsman doesn't put sugar on his porridge. So we have a test for true Scotsmanship.
But what is the test for a Christian? Do we have inclusive and exclusive criteria?
According to Wikipedia there are 2.4 billion Christians on earth. What do they have in common?
What do the other 5.4 billion not have?
Would your answer significantly change that number of 2.4 billion?
However, I've been around Christians long enough to have an appreciation for the idea that the Nicene Creed defines the boundaries of Christian belief.
I do not accept the idea that bad behavior means you are not a Christian. You can sin and still be a Christian. Happens all the time. I think that those who engage in the "They aren't a real Christian" are giving in to the "No true Scotsman" fallacy.