Has an atheist ever accused you of committing the infamous No true Scotsman fallacy? It usually comes when an atheist cites atrocities performed by some Christian, and then the Christian responds: “No true Christian would be murdering people like that.”
Here is a simplified illustration of the alleged fallacy:
McDonald: “No Scotsman would murder someone.”
Antony: “But wait, we know a Scotsman just murdered someone yesterday!”
McDonald: “Well no true Scotsman would murder someone.”
Granted, McDonald is seeming to engage in fallacy, but that is for the following very particular reason: there is nothing in the concept of an individual's being Scottish which stands in tension with the individual's murdering someone.
Now sure, there would be a problem if your dialogue partner was adding in that criterion to “being Scottish” in an ad hoc way, but it is confused to think that a person's adhering to view X cannot ever stand in tension with a particular behavior Y.
Hamish “No humanist would be a serial killer.”
Flew: “But wait, that one humanist serial killer was caught just yesterday!”
Hamish: “Well no true humanist would be a serial killer.”
Is it so clear that Hamish here is committing a fallacy? I don't think so. It is doubtful that Hamish literally meant that exceptions to his claim were metaphysically impossible. In a casual conversation like this, Hamish surely just means that:
“Engaging in serial killing strongly goes against the principles of humanism, so much so that a person's acting that way is evidence that they do not really share those values, i.e. that they are not really a humanist. The humanist qua humanist is not going to be going around murdering people.”
You can replace serial killing with your favorite example. Either way, we can now similarly see that when a Christian casually says “No true Christian would be murdering people,” they surely mean something like:
“Murdering people strongly goes against the principles of Christianity, so much so that a person's acting in that way is evidence that they do not really share those values, i.e. that they are not really a Christian. The Christian qua Christian is not going to be going around murdering people.”
What's wrong with thinking this?
I believe the No True Scotsman fallacy, like some other alleged fallacies that get thrown around, is just an excuse for being uncharitable. It does not even deserve a name. If I see an atheist using this on a Christian, I just say “Look Mr. Atheist, you know what she is saying; she is saying that the doing of X goes against the principles and values of being a Y, and the doing of X can at least be evidence that they are not really a Y. You can disagree, but you're just being uncharitable right now.”