Found this on plebbit and it seemed helpful:
No True Scotsman is committed when you try to use a criteria irrelevant to the category in order to exclude someone from the category by creating a new category using the modifier "true." It's often used as a rescuing device when confronted by a fact that the person doesn't like.
For example:
Person A: "No Scotsman would be caught dead wearing pants instead of a kilt"
Person B: "But my friend Sean was born in Edinburgh, and he wears pants."
Person A: "Well, if he doesn't wear a kilt, he isn't a "True Scotsman.""
One of the difficulties in discussing this fallacy is that there is a difference between a true Scotsman and a "True Scotsman." What I mean is that a Scotsman is a person born in or naturalized to Scotland, and in that sense is true. That's just a Scotsman, no need to append "True". However, when talking about members of a religious community, you run into the problem of conflicting standards of membership.
For example, let's examine a few definitions of "Christian":
A person baptized into the Catholic Church.
A person who is a member in good standing of a Christian church.
A person who has asked Jesus into their heart and repented of their sin.
Those aren't the only definitions, there are others. However, you can see how a person who holds to definition 1 might exclude people who don't meet that standard, even though they do meet the criteria for 2 or 3.
The big thing that marks a "No True Scotsman" fallacy, in cases where there is a conflict of definitions, is whether the person using it is being consistent with their own definition.
Lets say an evangelical says that a woman who has an abortion is "No True Christian". In this case, they are being inconsistent with their own definition. The evangelical definition is going to be something close to 3, which means that this evangelical is now adding an irrelevant criteria to their already stated definition to try to exclude this woman.
One more point I'd like to add is that the presence of conflicting definitions does not lead to the conclusion that there is no proper definition that excludes some of the others. One of the reasons this fallacy is sometimes claimed inappropriately is that, in a case of conflicting definitions, the word "true" may be used in order to draw attention to the one the speaker honestly believes is the only proper definition. This is not fallacious use, fallacious use is a dishonest exclusion due to irrelevant criteria.