Tom: No true Scotsman drinks Jack Daniels.
Sally: My uncle Angus is from Glasgow and he drinks Jack Daniels.
Tom: Then he is no true Scotsman.
The above type of false logic is called the no true Scotsman fallacy. Here is an example where the fallacy is not present:
Anne: True Scotsmen comes from Scotland.
Ben: I know someone from Canada who loves Scotland, often go there and calls himself a Scotsman.
Anne: Then he is no true Scotsman.
The latter example is not the No True Scotsman fallacy, as “A Scotsman is a man from Scotland” is an entirely just definition of Scotsman. And this one?
Joe: Many Christians have been responsible for henious crimes in history.
Melissa: Then they are no true Christians.
Wether this is the No True Scotsman Fallacy, depends on the definition of “Christian.”
However you choose to define Christian, the definition most certainly is not “anyone who calls himself a Christian, is a Christian.” We don’t use that definition for anything else. We don’t believe that everyone who calls themselves “honest” are. We don’t believe that everyone who calls themselves “not overweight” are not. You cannot be a king, or a genius, or a dog, or a tall person, by calling yourself that. (If it worked that way, it would have been a very strong temptation to call myself drop-dead gorgeous.)
Simple word etymology is more useful: Christian has the root word Christ and the suffix -ian. A Christian is a Christ-following/ Christ-imitating person. Who is meant when we speak of Christ? He is the Jesus described in the New Testament, as described there.To be a Christ-ian, you need to follow/ imitate Jesus as he is painted in the Bible. That is where He is painted as the Christ.
When is “but they were not true Christians” a case of the no true Scotsman fallacy, and when not? Whenever an action is inconsistent with Christ and his teachings, it is entirely logical to say that this was not a Christian action. To claim, for example, that Christians do not kill people because they believe differently, is entirely just. Christ’s words on the topic is found in Matthew 13:24-31 and :36-43
A Christian who kills those who believe differently does it despite, not because of Christianity. It is possible to say that (s)he was a true Christian, but not to blame Christianity.
At least that’s how I understand it.