I’ve encountered the “not a true Muslim” retort many times. The arguer first makes a general statement about all Muslims, and then reframes it in order to somehow render it immune from any refutation. This is an informal fallacy known as the No True Scotsman fallacy, first described by the philosopher Antony Flew, who asked us to imagine a Scotsman shocked after reading a story in his morning paper about the action of an English sexual predator on the loose. “No Scotsman would do such a thing,” he tells himself. The next morning he sees another story about an even more heinous sex crime, this time committed by a Scotsman. Not ready to admit he was wrong, he says, this time, “No TRUE Scotsman would do such a thing.”
Ali Rizvi, “The Atheist Muslim” (St. Martin’s Press, New York), p49.