Comment by sjducb

This sounds like the No True Scotsman fallacy.

We know all true scotsmen are good upstanding citizens. If you find a Scotsman who is a criminal, then obviously he is not a true Scotsman.

If you find a rationalist who believes something mad then obviously he is not a true rationalist.

There are now so many logical fallacies that you can point to any argument and say it’s a logical fallacy.