No true Scotsman is a story used by the philosopher Antony Flew to illustrate a very common fallacious argument, often used by apologists to take advantage of the ambiguity of the definition 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 (who is from Scotland) puts sugar in his porridge.
Scotsman A: Aye... but no true Scotsman puts sugar in his porridge.

The implication is that Angus is not a true Scotsman, despite the fact that he is from Scotland.  The fallacy lies in redefining the word "Scotsman" in order to exclude those who put sugar in their porridge.

Similarly, apologists argue that Christians are good people by categorically denying that anyone who does a bad deed is a "true Christian".  The lack of a generally accepted definition of "Christian" allows apologists to redefine the word to fit their arguments.  For this reason, many self-professed Christians who commit bad deeds are excluded from the group by apologists.

Since the Scotsman fallacy relies on ambiguity in the definition of the word "Scotsman", it is a form of equivocation. It is also Moving the goalposts.