Here is part of the point that Cerpin is trying to make with his No True Scotsman fallacy.
Religious people only selectively see what they want to see within their religion. If they are good people and believe their religion is all good, then that is what they will selectively see. When they see an ugly wart on their religion they fail to process that and write it off as something else.
The thing is, Christianity as a religion is defined by the Bible, however many times it has been re-written or added to in the case of Mormons for example. The religion of Christianity is what is laid out in their little book. The problem is that book is filled with good stuff and bad stuff when it comes to "teachings" for humans. When you have two people read the book and what teachings to go off of they have several different things they can follow. They can follow only the good, or what they perceive to be good. They can follow only the bad thinking it's good. Or they can follow it all because it all has to be good.
Those that are following the good but see someone following the bad write them off as not a "true" Christian because they selectively see only the good.
But the Bible, like damn near all religions thus far to date, are filled with hate, death, war, rape, slavery, and other morally questionable activities. When you get someone that gets an idea into their head like say Hitler, that believe he is the new Christian champion that is doing God's will by eradicating all Jews, you later have ever Christian that will claim he was never a Christian. They see only the "love and peace" portions of the bible and think anyone that deviates from what the perceive as the "true" version of Christianity as non Christians.
Part of the reason that there are sooo many different sects of Christians and why they used to all fight and war with each other.