The "no true scotsman" fallacy

The usage of the so-called "no true scotsman" fallacy can be a bit hard to understand as an actual argument. (In other words, it can be hard to understand how it could be used in a serious argument.) After all, it just sounds like a comical quip, rather than anything said seriously.

The classical rendition of the fallacy is as follows:

Person A: "No Scotsman puts sugar on his porridge."

Person B: "I am Scottish, and I put sugar on my porridge."

Person A: "Well, no true Scotsman puts sugar on his porridge."

There is, however, a form of this fallacy that's surprisingly often used in Christian apologetics, most often by those who claim that Christians are different from other people thanks to the fact that the Holy Spirit is affecting them.

For example, a common claim is that Christians would never mass-murder people. Now if you bring up a counter-example ("this person was a devout Christian, and he mass-murdered people") the standard answer is the fallacy in its purest form: "He wasn't really a Christian."

This is basically a circular argument that's used to dodge any possible counter-arguments to the claim. The claim becomes basically impossible to prove wrong because any counter-example you may bring up will be countered with a simple "he/she wasn't really a Christian". It doesn't matter how deeply Christian the person might have been, it doesn't count if he was a mass-murderer.

The condition "does not commit mass-murder" (or any of the myriad other such claims) is basically implicitly added to the definition of "Christian", and thus anybody who does not fit that definition is thus "not really a Christian".

Ultimately this dodges the burden of proof for a claim such as "Christians never mass-murder people" (due to God acting upon them or such) because the definition of the term is basically circular.

Why is this a form of circular argumentation? Consider it like saying "no dog is white", and if a white dog is shown to you, just dismissing it by saying "that's not really a dog." And why isn't it a dog? Because it's white and, as established, "no dog is white", duh. The claim itself becomes the definition.