A common tactic used by unbelievers of various religions is to point to various evils in the world and claim that Christianity is responsible. It is claimed, for example, that the KKK are a Christian organisation. Christianity must be evil if it produced the KKK, right?
Answer: absolutely not. It is completely impossible to classify any group founded on racial hatred as Christianity. But when it is pointed out that even if a person or group identifies as Christian, that doesn't mean they are, unbelievers immediately cry "NO TRUE SCOTSMAN FALLACY!"
The no true Scotsman fallacy, also known as appeal to purity, is the ad hoc change of a definition to prevent the possibility of contrary examples. To put simply, if A cannot Y, then any As which do Y cannot be a true A.
The example from which the fallacy derives its name is as follows:
A Scotsman named Hamish reads a newspaper and reads about a serial killer. He declares that it cannot have been committed by a Scotsman because no Scotsman would ever do such a thing. As he reads on, he discovers that the murders were, in fact, committed by a Scotsman, so he declares that no true Scotsman would do such a thing.
Whether or not Hamish committed the No True Scotsman fallacy depends on the definition of a true Scotsman. Is it possible, for example, to call a serial killer a Scotsman if he was born in England, to English parents, and was in Scotland purely for the purposes of committing the murders? Answer: no. A true Scotsman requires some direct connection to Scotland, whether through residence or heritage.
In the image above, I have included an ant-like creature. However, while it looks like an ant, acts like an ant, and even smells enough like an ant to fool real ants, it is not an ant. It is a spider, myrmarachne formicaria (another similar species exists by the name of myrmarachne plataleoides). These spiders are, of course, not true ants. They mimic ants for the purpose of hunting them.
In the same way, the Bible talks frequently about "wolves in sheep's clothing". People who, though they pretend to be Christians (and may even believe themselves to be so), are actually heretics, con artists, and future apostates, who distort the word of God, and cannot be trusted any further than Jesus could have thrown the cross after the brutal beating He had received from the Romans. Now, as a general rule, the leader of a religion has full rights to decide who may and may not be classed as true followers. If a person fails to meet Christ's standard, it becomes perfectly legitimate to say they are not true Christians.
Thankfully, Jesus didn't leave us without sufficient record of His teachings. Christians today are privileged in that the word of God has been widely distributed throughout the world in many different languages. Even the most hard hearted unbeliever can read about what constitutes a true Christian as easily as he can read this article.
Therefore, the onus is on the unbeliever to defend their claim that Christianity is responsible for these great evils carried out by so-called "Christians". Typically, however, they won't try to do this. This is because the Bible can stand for itself. If an unbeliever can't build a straw man, they automatically lose a lot of ground. As soon as the Bible is put in its proper place as the sole and sufficient authority over the Christian faith, Christianity wins.