Yep, no true scotsman fallacy.
"No True Scotsman"
Argument: "No Scotsman puts sugar on his porridge."
Reply: "But my uncle Angus likes sugar with his porridge."
Rebuttal: "Ah yes, but no true Scotsman puts sugar on his porridge."
This form of argument is a fallacy if the predicate ("putting sugar on porridge") is not actually contradictory for the accepted definition of the subject ("Scotsman"), or if the definition of the subject is silently adjusted after the fact to make the rebuttal work.
The truth of a proposition depends on its adequacy to its object ("Is the drawing a true likeness of Antony Flew?"). The truth of an object depends on its adequacy to its concept ("Is the figure drawn on the paper a true triangle?"). Problems arise when the definition of the concept has no generally accepted form, for example when it is vague or contested.
"A true Scotsman" (a concept) is not on the same level as "a true triangle" (a concept) never mind "the true Antony Flew" (a concrete existing object). The formal similarity, "true X", and the corresponding feeling that the concepts should be on the same level, in some sense must be on the same level (even perhaps all exist as objects), motivates the fallacy. It is a short step from that feeling to treating one's own definition of a "true Scotsman" (who else's?) as having the same objectivity as that of a geometrical figure or an existing individual, and then attempting to make the world agree.
Using the context of culture, individuals of any particular religion, for example, may tend to employ this fallacy. The statement "no true Christian" would do some such thing is often a fallacy, since the term "Christian" is used by a wide and disparate variety of people. This broad nature of the category is such that its use has very little meaning when it comes to defining a narrow property or behaviour. If there is no one accepted definition of the subject, then the definition must be understood in context, or defined in the initial argument for the discussion at hand.
Some elements or actions are clearly contradictory to the subject, and therefore aren't fallacies. The statement "No true vegetarian would eat a beef steak" is not fallacious because it follows from the accepted definition of "vegetarian": Eating meat, by definition, disqualifies a (present-tense) categorization among vegetarians, and the further value judgement between a "true vegetarian" and the implied "false vegetarian" cannot likewise be categorized as a fallacy, given the clear disjunction.
In order for the "No True Scotsman fallacy" to apply, there has to first be the assumption that there is no such thing as an objectively defined "Christian", which is clearly not the case, except in the minds of those who do not know what a Christian is. Simply because a majority of people fail to define what is a "Christian" does not negate that there is an absolute object that exists.
Christianity has nothing to do with what someone does or does not do, but what they are. It is a distinct immovable identity that cannot be altered by opinion.
It is just as easy to define a "true" Christian as it is to define one that is not.
When somone claims that there are "former Christians", then they deny the objective reality of the identity of a Christian.