Imagine Hamish McDonald, a Scotsman, sitting down with his Glasgow Morning Herald and seeing an article about how the "Brighton Sex Maniac Strikes Again." Hamish is shocked and declares that "No Scotsman would do such a thing." The next day he sits down to read his Glasgow Morning Herald again and this time finds an article about an Aberdeen man whose brutal actions make the Brighton sex maniac seem almost gentlemanly. This fact shows that Hamish was wrong in his opinion but is he going to admit this? Not likely. This time he says, "No true Scotsman would do such a thing."
Why bring this up? It seems that NTS has become the accusation du jour for atheists/fools to hurl at Christians. A typical exchange will go a little something a-like-a this-a.
Atheist: There are many Christians who are evil, wicked, immoral, reprehensible, etc.
Christian: Anyone who does such things has shown himself to not be a Christian.
Fool: A-ha! It's the No True Scotsman fallacy! By relying on such a fallacy you show the weakness of your position, and admit defeat! You lose, sucka!
So the question before us today is, is this a proper place to cry 'fallacy'? Let's break down the fallacy and see what exactly is being said.
What you have is group A with essential and necessary membership requirements; everyone who meets the requirements is part of the group, and everyone who is in the group meets all the requirements. In math proof speak, they are "if and only if". In the Scotsman fallacy, the requirement is "being Scottish" (whether born in Scotland, born of Scottish parents and/or just lives in Scotland, the exact definition is beyond the scope of this paper). A man born of Scottish parents in Glasgow is a Scotsman, a German born in Berlin or New Zealand is not. For the group "Pitchers for the Atlanta Braves", the requirements would be (1) a Major League Baseball player (2) for the Atlanta Braves (3) who plays the position of pitcher. Anyone who meets all three requirements is in the group, anyone who meets zero, one, or two isn't. Clear? Mmmmkay.
Next, you have some separate, unrelated issue B, usually a negative condition, such as the acts of the Aberdeen man in the example above. Finally, you put them together - someone who meets all requirements for A, but also does negative B. Is he still in group A? Of course. But someone else in group A could say no, which would be the fallacy.
Now how does that apply to the exchange with the atheist fool? In this case, group A would be "Christians", and issue B would be wickedness/immorality. His claim is that when the Christian asserts that wanton, unrepentant sin reveals someone to not truly be a Christian, he is committing this fallacy. Does this hold up? In order to work, the fallacy requires B to be a completely separate issue from group identity. In other words, whether or not someone is so evil that even an atheist is willing to call it evil must be completely unrelated to whether or not he is actually a Christian.
But if something like, oh, say, submission to the Lordship of Christ is a necessary requirement of Christianity, the objection completely falls apart. (See, among numerous examples, Mat 7:15-27, Romans 6, like, all of 1 John, etc etc.)
A few closing observations.
1) I have no idea how the no-Lordship folks would respond to this. But that's way down on the list of objectionable things about no-Lordship theology, so whatever.
2) Take a gander at the introduction to John Piper's excellent little book, Finally Alive (pdf). Go ahead and read the rest of it too - but for now just the intro will suffice. Note that both the atheist fool and the Barna group begin with the same premise: that someone professing to be a Christian or born again actually is, and that his behavior has no bearing on the validity of the claim. To them, the only requirement for being in A is claiming to be in A. Not surprisingly, both reach a similar anti-Biblical conclusion (that being born again does not result in sanctification). Just something to keep in mind when reading the next Barna book about how we need to reform the church or church growth wizardry or whatever.