Orthodox Christianity is considered to be what comes from the Old and New Testament but many denominations/religions call themselves Christians without adhering strictly to the Bible as we know it today. I am specifically thinking of Mormons and by some perspectives, Jehovah's Witnesses. I am sure there are countless others as well.
Is there a definition of "Christianity" or for a "Christian" for which one can objectively compare and contrast against to determine if a belief or a person is "Christian"?
For example, (please don't argue for/against any of the points in this paragraph, it is mean purely as an illustration) one may say Roman Catholicism is Christian because they believe Jesus is God. Mormons believe Jesus was just a normal guy that became a god. Islam believes that Jesus was just a uniquely pious guy. Atheists believe that Jesus was just a guy.
I think most people would agree that Catholics are Christian and Atheists aren't. So then the question is this: Where is the line between "Christian" and "non-Christian"? Is it even possible to define one in an objective sense?