No true Scotsman fallacy
"No true Scotsman" is story used to illustrate a very common fallacious argument, often used by apologists to take advantage of the ambiguity of definitions of a certain key word (or words) in their argument.
The classic story goes something like this:
- Scotsman A: "You know, laddie, no Scotsman puts sugar in his porridge."
- Scotsman B: "Is that so? I seem to recall my cousin Angus puts sugar in his porridge"
- Scotsman A: "Aye... but no true Scotsman puts sugar in his porridge."
The implication is that since Angus puts sugar on his porridge, Angus is not a true Scotsman by definition, even though he (presumably) comes from Scotland. This is playing fast and loose with the definition of "Scotsman".
In a similar fashion, many apologists try to prove that all Christians are good people by categorically denying that anyone who does a bad thing is a "true Christian". Unlike the word "Scotsman," there is no generally accepted definition of the word "Christian," so you can pretty much define it however you want. A very inclusive definition might be "Anyone who claims to follow the religion of Christianity." A very exclusive definition might be "Only those people who precisely practice the sect of Christianity that I agree with."
Obviously there is a lot of wiggle room between those two extremes. Since the Scotsman fallacy relies on ambiguity in the definition of the word Christian, it is a form of equivocation.