The definition of a True Christian depends entirely on whom one is talking to, but it would also appear that in essence, every Christian believes they are the real deal True Christian. Every Christian believes they hear directly from god, that they are inspired and lead by the Holy Spirit, and that they understand the message of the bible correctly, even where other Christians may err. Thus, if anybody claims to be a Christian but believes something different from them, they feel a sense of assurance in claiming the dissenter is not really a True Christian. Whenever atrocities committed by Christians in the name of god are mentioned, for example, the apologists tell us that those perpetrators were not really followers of Christ after all, they are not True Christians, that they were appropriating the Christian message for their own ends and that their warped view of gospel tenets says more about their inherited sinful nature than the true teachings of God. This argument would appear to imply that there is one very simple, unequivocal message within Christianity which is lost on all but True Christians. The question of, "What is a True Christian?" is one that should be able to be answered in one sentence. But it rarely is, because a one-sentence answer both at the same time pigeonholes people into an undefined set of beliefs and opens the door wide to allow too many different beliefs into the mix.
If Christianity is really as clear cut and as straightforward as some Christians would have you believe, then how do divisions in basic beliefs even manifest themselves? Why is there more than one denomination of Christ? If there really is only one true, clear message in the bible, then it should be impossible to distort or pervert that message in any way without it being clear and obvious to anyone who clearly follows that same message. However, when Christianity is examined objectively, and even subjectively, we find that contradictions are rife: what God commands in one chapter, he condemns in another, and teachings upheld by one group are completely ignored by others. Thus, if the bible does have only one message, it is one very mixed and unclear message, open to unique interpretations.
Christians cannot agree on even the most basic of beliefs, such as the relevance of the
Saturday Sabbath, alcohol consumption, painted faces or not, pre and
post-Tribulation rapture, predestination, or the “correct” way to interpret the prophecies of Revelation and the importance of faith over works. These disagreements, and many others like them are so divisive that they result in other denominations being formed since the Christians caught in the middle can no longer agree and “fellowship” together. As a result, Denomination A becomes convinced that they have the one true and only correct scriptural interpretation and that Denomination B is completely wrong and, by extension no longer following the teachings of Christ. In other words, not really True Christians.
Apostates every one of them! Conversely, Denomination B thinks the same of Denomination A. Don't get me started on Denomination C, D and E. See where this is going? Where is the unity? Where is the consistency? Why so many different versions of the One True Way?
Credibility gets stretched, credulity rules when everyone in all of these Denominations are saying the same exact thing about themselves and the Others. It's highly unlikely they are all correct. Impossible, actually. But it's quite possible that one, and only one, is correct. They all believe they are that One.
"Man is the only animal with the One True Religion - several of them." - Mark Twain.