This is a favorite approach among Christian apologists. We're told that Hitler was a Christian and look what he did. We respond with "no true Christian would do such a thing." We are pointed to the horrors of the Crusades and respond with "no true Christian would do such a thing." Same kind of reasoning.
But let's consider it for a moment. First, is it an actual fallacy? While it certainly can be, I would argue that it isn't always. Here's the notion. There are two sets. One is the set of people in Set X. The other is the actions of Set Y. Restating the fallacy in my new set terms, "no true member of Set X would perform the actions of Set Y."
In terms of the logical statement:
1. No one in Set X does something from Set Y.
2. He does Set Y.
Therefore, he is not a member of Set X.
The suggestion from the term "logical fallacy" is that the argument is not valid, but is such a claim ever valid? Well, if membership in Set X is defined as not performing the actions of Set Y, then it would be true and not a fallacy. It is said that the slippery slope fallacy is only a fallacy if it doesn't happen. The same is true for the "no true Scotsman" fallacy. As long as the definition of "true Scotsman" accurately excludes the activity in question, then it is not a fallacy. In this case, this logical argument is only a fallacy when the premise fails. That is, if it is true that "No one in Set X does something from Set Y", then it isn't a fallacy. If, on the other hand, that premise is false, the argument is false.
So, if we're talking "true Scotsman", consider the following. Let's say that Addison Macalister of the clan Macalister is talking with Ralf Bauer. Ralf is telling Addison, "I am of strict German heritage, to be sure, but I'm a true Scotsman at heart." Addison would not be in error if he said, "No true Scotsman is of German heritage." That's because "true Scotsman" has a definition that excludes "German heritage".
So back to this approach from a Christian apologist perspective. Is it a fallacy? I would argue that it can be and, if we're not careful, it often is. I've heard, "No true Christian would ..." and have it end with "vote for Obama" or "be a Democrat". This is simply false. For whom you vote or with which political party you side is not part of the definition of "Christian". That is fallacious. Nothing in the definition of "Christian" requires perfection. Indeed, we understand that no one, Christian or not, arrives at perfection in this life.
"Well, then," our opponents would say, "you are back to the problem of Hitler and the Crusades, aren't you?" Not at all. You see, there are two other factors to consider. First, one argument would be "Hitler was not a true Christian because he did these things." But a better statement would be "Hitler's actions did not express Christian values as represented by Christ (remember, "Christian") and the Bible." The former statement is questionable under the "no true Scotsman" fallacy. The latter is a statement of fact. The other factor, however, is found in the biblical definition of "Christian".
There are a variety of components that Scripture includes as "Christian". "You must be born again" (John 3:3-7) would exclude anyone who is not born again. "I'm a Christian but I'm not born again" would indicate that the statement is just as false as describing a "round square". Clearly faith is a mandatory component. An "atheist Christian" (such as a noted pastor I could name) is a violation of logic. There are, then, various pieces to "Christian" without which one cannot be defined as "Christian" despite their claims to the contrary. While "sinless perfection" is not one of those pieces (and, thus, it is false that "no true Christian would commit such an act"), here's what John does say on that:
No one born of God makes a practice of sinning, for God's seed abides in him, and he cannot keep on sinning because he has been born of God (1 John 3:9).There it is. While true believers can and do sin (1 John 1:8-10) -- and when they do they have an Advocate (1 John 2:1) -- what they cannot do (John's term) is to make a practice of, keep on, sinning. So if the claim is "No true Christian sins with impunity", the statement is fact, not fallacy.
The "no true Scotsman" fallacy is out there. Like the slippery slope fallacy, it is something to watch out for. Like the slippery slope fallacy, sometimes it is not a fallacy. If you fancy yourself a Christian apologist -- a defender of the faith -- it would likely be wise to keep it in mind and be careful not to do it. Indeed, Christians -- true Christians -- fail miserably. (Look at Peter as a prime example.) But remaining within the biblical definition of Christian and calling those outside that biblical definition "not Christian" is not a fallacy. Neither is recognizing activities in Set Y (which we would call "sin") as "not Christian". There are those who claim to be Christians who do not meet the definition of Christian, and there are activities that are deemed "unChristian" which are indeed outside of the bounds of good Christian activities. The fallacy exists. We should just be careful to avoid making that mistake.