I absolutely despise this phrase: "They aren't True Christians." People just love bandying it about left, right and center as it suits them and it has become a rather abusive, repetitive term, which both camouflages hypocrisy and vindictiveness as well as serves as an automatic conversation stopper and verbal passing-of-the-proverbial-buck. It is a dangerous and unintellectual way out of an argumentative corner.

The emotional connotations of this term involve the speaker immediately being put in a position of self-righteousness and can instantly be assumed by listeners to be the 'correct' one in the situation. From my experience with people, online and in real life, who have used this phrase, it is generally used when referencing someone who is not present at the time, adding the smear of gossip to the phrase. The person being labelled a False Christian, someone who has not interpreted the scripture properly (a grave insult to someone who no doubt considers themselves to be, in turn, a True Christian), is left in a position where they are not even able to defend themselves in an intellectual, theological debate about the nature of this serious term so flippantly launched around from Christian to Christian.

The divine implications are even more severe. All Christians naturally believe that they are True Christians, which makes Christians with different approaches and interpretations to Christianity bearing the mark of False Christianity. Even though False Christianity is not a term I have ever heard or seen it is still naturally implied when one Christian says to another Christian that they are not a True Christian. It is worrying to think of the gravity with which Christians apply this term to their fellow believers. There are already so many different denominations within Christianity, using such a divisive term only serves to separate them all even further from each other with the result that they all become very strong, antagonistic, opposing religions. When arguments involving this phrase develop online I have noticed immediate hostility from both parties involved, adding fuel to the already dangerous fire of religious zeal. If you believe so strongly that not just your religion and your god but also your mere interpretation of something puts you in a more divine, self-righteous, self-justified situation than another believer in the same religion or sect as you, then how do you police your own morality within such a system? Surely if you are a True Christian you have a so-called hot-line to God and can decide for yourself what is right and wrong, since you are on his good side and are the favourite. The logic seems entirely selfish, warped, egocentric and, ironically (although I say this hesitantly) unChristian.

Here are some examples of where I saw this phrase used most recently, if you are interested in some context surrounding its use:

Lexis1234 YouTube comment http://ht.ly/1UVFE on a video by Matthatter (spelling and grammatical errors edited out for continuity):
"Lauren, may you continue to follow the one and only TRUE God of the Bible and not the god your family tried to brainwash you into thinking exists. I pray a True Christian church will open their hearts and their arms to you and embrace you. Praise God that you left them. Jesus said "I have come to turn daughter against mother, son against father" because that son or daughter will follow the TRUTH, THE WAY, AND THE LIGHT."

In the film Agora (a movie about the philosopher Hypatia and the fanatical Christians who destroyed the legendary Library of Alexandria I highly recommend), one Christian (Synesius) says to another Christian (Orestes) that he is not a True Christian (this confrontation occurs 40mins into the second half of the film).

Google True Christian and you get 32,600,000 results in 0.16 seconds.
Google What is a True Christian and you get 33,600,00 results in 0.21 seconds.
Some of the more disturbing hits amongst these hundreds of thousands include links to the True Christian Ministries. When I see titled on Google such as, "if you vote for Obama you aren't a True Christian," and "Can you believe in Evolution and still be a True Christian?" I start to worry and my fundie-feelers start tingling. I don't think I need to explain why and how religion flies in the face of rationality and societal, not to mention scientific, progress. Even more worrying are websites which pose the question, "Can a True Christian fall from Grace?" If there is anyone in the world who believes that the answer to this is "no" in any way, shape or form, they need to be institutionalised immediately. If you consider yourself or someone in your community, whether a peer, family member, or religious figure of authority to be in a position where they are a True Christian and this makes them exempt from falling out of favour with God, there are some serious moral issues of social abuse, intellectual irresponsibility, and lack of accountability which need to be addressed.

There are also a myriad definitions of True Christianity available.
This article offers a more in-depth Christian analysis of who a True Christian is:

These inspired scriptures certainly show that most people and nations have been misled and deceived by Satan the Devil! This, very frankly, is the "key," which explains why there are so many obvious contradictions in our supposedly "Christian" society. It shows why this world is so mixed up and confused about what the word "Christian" really means. It shows that our civilization has not been following Christ, but has been deceived into going many different ways under Satan’s confusing influence!

For the true Christian, God becomes the center of everything. The genuine Christian is really God-centered. He will want to please God in all his thoughts and actions.

Under the subheading, A Planned Deception, in that article, there is a paranoid, egotistical breakdown of how a certain part of the scriptures have been misinterpreted.

People who do things with scripture others don’t approve of are readily labeled with this sarcastic, self-enhancing phrase, but what they are saying is in the scripture. It is an interpretation and surely, if you believe in a god of some sort, it is someone’s divine right to interpret things the way they see fit as is evident by the many denominations of Christianity in the world, not to even mention other religions and alternate religious texts. What honestly makes one better than another? Is it false simply because it isn't directly in line with what you were indoctrinated to believe as a child? Christians who do harm to others, whether physically or emotionally (such as the Westboro Baptist Church, for example), call arguably harmless Christians False Christians, and from what I have noticed it works the other way around too. So the issue of right and wrong is confused and the absolute morality professed by Christian believers becomes a blurry puddle of questionable morality and irrational, man-made, relative moralities designed to serve an immediate, contextual, very Earthbound purpose under the disguise of having a divine purpose.