A: No Scotsman drinks his tea with sugar.The problem is, of course, that whether one drinks tea with sugar or not is not really part of the definition of "Scotsman". Instead the concept is based on ancestry, place of residence, integration into a community of other Scots, etc. A is simply wrong unless he unilaterally redefines the word. Sometimes disagreements between two people are due to at least one side being wrong on the facts, sometimes they are due to different definitions of a relevant term (consciously or unconsciously), and sometimes one side is simply irrational and tries to twist the definition of a term to win by default. The latter two scenarios are the ones where the No True Scotsman becomes relevant.
B: That cannot be true. I know a Scotsman who does.
A: Then he is not a true Scotsman, because no Scotsman drinks his tea with sugar.
The relevance for religions or other belief systems is pretty obvious - many of us will have had a conversation like the following:
C: Christians are more moral than atheists / Without religion there is no moral guidance, and terrible things will happen.As in the case of the Scotsman, it is now important to have a clear definition of "a real Christian". Unfortunately, that is where it gets difficult.
D: Not so fast. What about the inquisition? They did terrible things to innocent people in the name of Christianity. And mind you, Adolf Hitler was a Catholic.
C: Well, none of them were true Christians. If they had really been Christians, they would not have done these things.
I have discussed a related issue with several people over the years, namely what exactly one would have to believe to still count as a Christian, and some of them have argued that everybody who claims to be one should be regarded as one. That, however, is a bridge too far. Surely a concept has to have some content to be useful for communication. For example, if somebody favors the privatization and deregulation of all economic activity, he has surely lost the right to call himself a communist. Words have meanings, as the saying goes. The same applies to the word "Christian". So what is it that defines that religion?
My own view is that there are two possible definitions, a narrower and a wider one. In the narrower sense, to be a Christian one would have to believe that there is one very powerful creator god and that Jesus is either his most important prophet or his incarnation or his son (whatever that means), and that his birth and death were events of cosmic and spiritual significance. Narrowing it further still would be a bit problematic because it would exclude numerous sects that were definitely part of the great diversity of Christian tradition but merely had the bad luck of losing out against what became mainstream over the centuries, as for example those streams of Christianity that consider(ed) Jesus to have been fully human.
In the wider sense, I would be willing to count everybody who is a "Jesus-follower", even if they are essentially agnostic or even atheist but believe (against all evidence from the gospels, I might add) that Jesus was an inspired moral innovator and teacher who should serve as a model for one's own philosophy of life. To me, Christianity is defined by Christ. Well, duh.
But that also means that it is not defined by being a good person. People can be mass murderers and still be Christians if they believe those above items. (As, again, Hitler did, from what is known. Apparently he considered Jesus not to have been a Jew but somebody who struggled against "the Jews", and he saw himself as a tool of divine providence. That may sound bizarre but Hitler's crimes aside it really isn't more wacky than the beliefs of many other people who call themselves Christians, e.g. those who assume that Jesus was blonde and blue eyed and preached in Jacobean era English.) So the considerations around the No True Scotsman fallacy apply with full force to this religion.
The same goes for atheism, of course. As far as I know, Stalin appears to have been an atheist, and because atheism is merely the absence of belief in gods and not necessarily "being a nice person", the responsibility for mass murder does not mean Stalin was No True Atheist. But note that the real point when these issue are discussed is a different one anyway: Did X do the deed because of their religious beliefs? And it seems quite clear that neither Hitler nor Stalin ordered atrocities because they were a Christian and atheist, respectively, but for different reasons.
Now finally to the word that featured so prominently in my previous post, the one that prompted me to write this: rationalist. I should first stress that I do not use the word in its narrower philosophical sense, as the stance that pure reason is the primary or only way of generating knowledge, as opposed to empiricism. Instead, what I mean is the more modern, wider concept in which the use of reason and of empirical data (rationality s.str. and empiricism) are combined while trying to avoid appeals to authority or tradition.
The word has a bad reputation in some circles where it is associated with a cold, uncaring approach that exploits nature and fellow humans. There are also those who point towards self-declared rationalists, note that those people did or believed stupid and harmful things (e.g. racism, social darwinism, etc.), and conclude that rationalism leads to those things or at a minimum is no safeguard against them. Here is the question: Me being a rationalist, can I say that all of that is Not True Rationalism, or am I simply committing the same fallacy?
I would argue: partly the first, partly the second. The rationalism in the negative, technocratic sense is indeed an application of true rationalism, but the complaints about it miss the point. It cannot be reasonably claimed that rationalism alone can do any more than help us find out what is correct and what is false. It cannot tell us what we should do with that information; that is where Hume's "reason is a slave to the passions" comes from. What would a perfectly rational mind without values and emotions do? Well, nothing, or anything, because it would have no way of even preferring its continued existence over its annihilation. You have to add values to the mix to act on the knowledge that the use of reason and evidence allows you to generate. But conversely, that also means that rationality is only a highly useful tool which can be used for any purpose, and thus not any more guilty of what it is used for than a hammer. In that sense it is again different from any religious approach because the latter are not only approaches to generating beliefs, they also come pre-equipped with non-negotiable very specific beliefs (such as that there is a god).
On the other hand, I would argue that an English gentleman from 1860 or a German professor from 1930 claiming to be only rational when concluding that non-whites are inferior to whites really got it wrong. They were Not Truly Rational, and to say so is not fallacious, because being rational means to follow the evidence where it leads whereas they were clearly captive to the prejudices of their times. To a degree that is understandable, of course, and the same is true for all of us today. Anybody who claims to be fully rational is deluding themselves; rationality it is something to strive for, not something to show off like a badge of honor.
It is important to try to be as rational as possible because the more accurate our picture of the world and of our place within it are, the better are the decisions going to be that we make. But it is unrealistic to expect that one can always get it right, and that is also why those racists of previous eras aren't really an argument against valuing reason and evidence over faith. We don't consider walking to be useless or harmful just because we trip from time to time, right?