The diverse nature of Christianity is a fundamental flaw to the claim that it is a true religion. Christianity is a diverse religion. It is composed of a vast array of different denominations each with their own take on what Christianity is or isn't and although granted most of these differences are relatively trivial in some cases there are disagreements over fundamental aspects of the religion.
Once a month I attend a local atheist get together and we sometimes get a few Christians attending. One evening the conversation drifted onto the subject of the trinity and it just so happened that one of the two Christians that had come along didn't believe in trinitarianism. This sparked off quite a lengthy debate between the two Christians and a few atheists and whilst I really appreciate it when anyone that believes in a God comes to our get togethers I couldn't shake the thought that we might as well be arguing about the colour of the Loch Ness monster.
The problem that the Christians were coming up against is that there simply isn't a method for establishing who, if anybody, has actually got it right. The Christians that came to our little get together may have identified as Christians but clearly they believed in two fundamentally different types of God.
I see this same issue happening on Internet forums. Christian beliefs are wide and varied. There is no possible way that they can all be right, it is worthwhile pointing out again though that it is entirely possible that they could all be wrong.
Considering all this how then can Christianity be described as a true religion? How can we call something true when there are numerous different versions of it, many of which are mutually exclusive from each other and there is no methodology for determining which version is the correct one? If Christians themselves are not capable of deciding what Christianity actually is or isn't then I don't see how it is even possible to consider it true.