No true Scotsmen – or Christians

The “no true Scotsman” fallacy (as it is called) is illustrated like this:
William: No Scotsman wears underwear under his kilt
Angus: Not true! I’m a bonnie Scotsman, and I wear underwear under me kilt!
William: Ach! Then yer nee a true Scotsman! Away with ye!
One who uses this fallacy makes their position beyond critique by [...]
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