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".This comes up a fair bit when dealing with abstract ideas that are imprecisely defined. You can play with the definitions and make it unfalsifiable and still get away with it rhetorically. Consider a dialogue like this:
Protestant: Evangelical protestants agree on all important matters of doctrineSo what started as an impressive statement about the unity of evangelicals ends up being not quite so impressive. Like Hamish McDonald the evangelical thinks he is making a true and meaningful statement. One has a notion of a true Scotsman in his mind. The other has a notion of a true evangelical in his mind. They both want to convince themselves that the community they consider themselves a part of does not have a problem. Scotland does not have a crime problem. Evangelicals don't have a heresy problem. The trouble is they are wrong. Any reasonable look at the data shows they are wrong.
Catholic: They don't agree about the Eucharist
Protestant: That is not an important matter
Catholic: What about Rob Bell? He denies the doctrine of hell.
Protestant: Rob Bell is not an evangelical
With the evangelicals what has been happening really since the reformation but in an accelerated way over the last 100 years or so is constant change in doctrine. They have responded in two ways. They have declared many doctrines to be unimportant. and they have declared may groups of people to not be Christian. So they agree with fewer and fewer people about less and less. But none of this is precisely defined. It can't be. Every protestant has a different notion of what it means to be Christian and who has rejected some essentials of the faith. So being a Christian is like being a True Scotsman in the sense that what it means can change over time and change from person to person and really becomes just a vague notion in people's heads.
Against all this we have the doctrine of infallibility. It is very precise. This is the Catholic faith. People don't agree with it? So what? It does not depend on anyone's agreement. It is falsifiable. All you would have to do is find one logical contradiction in what is defined infallibly. But most of all it shows that the Catholic faith has the character of truth. That is it does not change from person to person. It transcends time and culture.