Can Christianity be objectively defined? 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?
As already discussed on meta.C.SE here and here, the short answer is probably "no", except to say that one who identifies himself as a Christian is a Christian.

Of course the long answer depends on the perspective of the person answering the question.  It is for this reason we have essentially decided this question is off-topic on this site.



And as an aside, let me add what might be some consolation for anyone who does not like this answer:


  “If your brother or sister sins, go and point out their fault, just between the two of you. If they listen to you, you have won them over.
  Matthew 18:15 NIV


In this context, brother or sister means other Believer. Christians have a responsibility to hold all other Christians accountable for their actions.


  Take special note of anyone who does not obey our instruction in this letter. Do not associate with them, in order that they may feel ashamed. Yet do not regard them as an enemy, but warn them as you would a fellow believer. 
  2 Thessalonians 3:14-15 NIV


As this "this letter" was written to Christians, we can also draw that the context here is other Christians.

Other verses say similar things, about holding other Christians accountable.

What this means is, we, as Christians, have a responsibility to hold all other professing Christians (whether they pass our litmus test or not) accountable for their actions.

Was David Koresh a Christian? I suspect he would not have passed the litmus test of many people on this site. However, the fact that he considered himself a Christian is enough that, if any of us had conversed with him, we could have done so in the context of two Christians.