If there's one thing atheists love more than a straw man, it's a living straw man. There are many people claiming the name of Christ who have some very weird, wacky, and even wicked beliefs. Thankfully, it is rather easy to denounce a heretical belief as being not Christian. This, however, is when the atheist replies with accusations of the No True Scotsman fallacy.
The No True Scotsman Fallacy occurs when someone selectively defines X in such a manner as to exclude inconvenient or disconfirming examples. "No Scotsman would commit a murder." "This Scotsman committed a murder." "That Scotsman is not a true Scotsman." However, the fallacy does not occur when there is something in the definition of X that really would exclude the inconvenient example.
I ask it, what exactly is a true Scotsman? A Scotsman, by definition, has some kind of connection with Scotland. We could argue about how strong that connection must be. Perhaps all you need is a Scottish relative. "I'm a true Scotsman because my great great grandfather was Scottish." Or perhaps it needs to be a little stronger. Perhaps you must have a Scottish citizenship. Whatever the case, I think we can all agree that someone with absolutely no detectable ties to Scotland would not qualify as a Scotsman. Take, for example, Irrfan Khan. Irrfan Khan is an Indian actor. To my knowledge, he has no direct ties to Scotland. It would not be the No True Scotsman fallacy to say he is not a Scotsman, even if he started running around claiming to be a Scotsman.
So now we can ask what is true Christianity? Christianity is both a very inclusive and very exclusive faith. It is very inclusive because one does not need to be perfect in order to be Christian. You don't need perfect doctrine, and it is a basic Christian belief that perfect behaviour is impossible. You cannot say something like "no true Christian believes in Evolution", because even though Evolution is out of step with Christian doctrine, Creationism is not an essential doctrine. Similarly, you cannot say "no true Christian kills people", because although it is certainly out of step with Christianity to kill, Christians are still human, and thus still very much capable of terrible things.
However, first of all, you can say "no true Christian believes Jesus is dead". Christianity literally revolves around the resurrection. If Christ is not risen, there is no Christianity. Christianity necessarily excludes the rejection of its core doctrines, and by extension excludes those who do reject them. Furthermore, even if someone is a genuine Christian, yet rejects a non-essential doctrine, or accepts a heresy, you can nevertheless call the belief not true Christianity. On top of this, Christianity places a heavy emphasis on the link between faith and works. It is entirely possible to sin as a Christian, but the question must be asked why? If someone lives in persistent sin, it becomes more and more possible to not only denounce the act as not Christian, but sometimes also the person.
All of this makes it completely impossible to pin the crazier "Christian" beliefs on Christianity, and non-fallacious to block attempts to do so. Put simply, Christianity is based on the teachings of Christ. Christ did not start 900 Churches, He started one Church, with a single unifying document, commonly called the Bible. In it, we find the very words of God. These are the beliefs Christ intends us to hold, and anything that does not fit with it is, by definition, not truly Christian, even if it claims to be.
The main doctrine of Christianity is the Gospel. The Gospel is a doctrine which states that all have sinned and fallen short of the glory of God, and the wages of sin is death. However, the free gift of God is eternal life, given by grace and received by faith. Those who confess Jesus as Lord and believe He rose from the dead will be saved.