Also known as a form of “Appeal to Purity”, No True Christian and a form of No True Crossover Fallacy
In this form of faulty reasoning one’s belief is rendered unfalsifiable because no matter how compelling the evidence is, one simply shifts the goalposts so that it wouldn’t apply to a supposedly ‘true’ example. This kind of post-rationalisation is a way of avoiding valid criticisms of one’s argument.
All X are Y.
(the claim that all X are Y is clearly refuted)
Then all true X are Y.
Angus declares that Scotsmen do not put sugar on their porridge, to which Lachlan points out that he is a Scotsman and puts sugar on his porridge. Furious, like a true Scot, Angus yells that no true Scotsman sugars his porridge.
Flew, A. (1984). A Dictionary of Philosophy: Revised Second Edition. Macmillan.