The "No True Scotsman" fallacy is a bane to Christian apologetics, because Christians get accused of making it far more often than we actually make it. Anti-Christians, being faced with such a robust faith, have a much easier time when they can just attack the "living straw men". That is, heretical Christians who have nothing to do with the Christian faith, but are much easier to refute than actual Christianity.
The "No True Scotsman" fallacy occurs when X (in this case, X would be Christianity) is selectively defined to exclude inconvenient examples. For example:
C: No true Christian commits murder.
A: This Christian committed murder.
C: That is not a true Christian. He committed murder.
This genuinely would be the No True Scotsman fallacy. However, there is nothing in the Bible that says a Christian cannot commit murder. In fact, it gives examples of the faithful who did some rather atrocious things, including murder.
However, the very rules of language absolutely requires that not every alleged instance of the NTS fallacy be correct. Christianity, you see, is not an ambiguous term by any means. The faith takes on the name of its founder, Jesus Christ, and the term was first used in the city of Antioch, to refer to His disciples. This means anyone who is not a disciple of Christ, however much they may claim to be, is not a Christian.
This includes the at least two people in my life I have met who claimed to be Christians, yet at the same time claim to be atheists. It takes nearly no education to spot the problem with this. Christianity and atheism are, by definition, mutually exclusive. You can't be a disciple of a God you don't believe in.
Let's look at things from a legal perspective. It is possible to be a police officer. It is possible to be a criminal. It is possible to be a police officer and a criminal. Let's look at two scenarios.
C: No true police officer commits a crime.
A: Todd is a police officer, and he committed a crime.
C: Todd committed a crime, therefore he is not a police officer.
This is the No True Scotsman fallacy. Nothing in the definition of police officer prevents a police officer from committing a crime. Of course, if a police officer is caught committing a crime, the odds are they won't be in the force for much longer, but as it stands, it is possible for a police officer to commit a crime and still, at least until their dismissal, be a police officer. Scenario 2:
C: All true police officers work in the police force.
A: Rob is not in the police force, yet claims to be a police officer.
C: Rob is impersonating a police officer.
Is this the No True Scotsman fallacy? No. Rob is not a true police officer. There are reasons to impersonate a police officer, be it as innocent as fancy dress, or as evil as gaining access to someone's house to burgle it. Neither of these scenarios involve real police officers. If you get robbed by some dude in fancy dress, that has no reflection on the actual police force.
In Christianity, heretics are like fake cops. Using their actions or beliefs as an argument against Christianity is foolish. Furthermore, it is often quite ironic. Of course, anti-Christian is quite a broad term. We could be talking about a Muslim, or a Hindu, or even, ironically, a heretical Christian sect. But the ones who most accuse Christian apologists of the NTS fallacy for dismissing heretics are often atheists. Evolutionists, even. Yet, these tend to be the same people who will dismiss other atheists as genuine atheists. The opinions of Charles Darwin himself are often tossed out, as if his writings are so irrelevant to the modern Evolutionist narrative as to be worth less than the paper they were originally printed on. How is it that atheists are so capable of dismissing each other, yet when Christians say "no, Jesus taught X, therefore X is the Christian position, and Y is wrong", these same atheists say "that's the No True Scotsman fallacy"?
The fact is, the Christian faith is very clearly defined in the Bible. Just as it is possible to be a police officer and a criminal, it is possible to be a Christian and deviate from the non-essential doctrines of the Christian faith. (In this case, the doctrines themselves can still be dismissed as not Christian). However, just as one cannot be a police officer without working for the police force, one cannot be a Christian without following Christ. That is not a fallacy to say. That is a fact.
Of course, it's not a fact unbelievers particularly enjoy, because it is easier to refute heresies than the real Christianity. This is because the real Christianity happens to be true. It is a faith based on the testimony of people who genuinely saw the Risen Lord. The Lord who rose for you. When Jesus died, He suffered the penalty for your sin. Those who confess Him as Lord, and believe He rose from the dead, not only become true Christians, but also receive eternal life for this faith.