There is a logical fallacy known as the “No true Scotsman” fallacy. Here is how it is described in Wikipedia.
Philosophy professor Bradley Dowden explains the fallacy as an “ad hoc rescue” of a refuted generalization attempt. The following is a simplified rendition of the fallacy:
Person A: “No Scotsman puts sugar on his porridge.”
Person B: “But my uncle Angus likes sugar with his porridge.”
Person A: “Ah yes, but no true Scotsman puts sugar on his porridge.”
This fallacy is often used in debates about evolution and creation. The evolutionist claims that scientists believe in evolution. The creationist points out that a lot of scientists believe in creation and reject evolution. The evolutionist says that they are not true scientists.
Even Christians sometimes use this fallacy to dismiss other Christians with whom they disagree on doctrine. I was once engaged in an internet discussion with someone who believed that the only true Christians were those who believed in the five points of Calvinism. Anyone who rejected this belief was not a true Christian. (If you don’t know what Calvinism is, you can find out here: What is Calvinism? )