No true Scotsman fallacy
The story goes something like this:
"No Scotsman would ever put sugar on his porridge!" "But what about Angus McMutton? He puts sugar on HIS porridge." "Och! I meant no TRUE Scotsman would ever put sugar on 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.