Ex: I used to be a Christian, but I no longer believe.Is the ex-Christian right here or is there something he is missing?
C: So you were never a true Christian?
Ex: I used to believe I was.
C: Clearly you were never a true Christian because true Christians are once saved, always saved.
Ex: But I used to believe I was.
C: But you are not now, hence you were never a true Christian.
Ex: That's an example of a "No True Scotsman" fallacy.
I believe it depends on which type of Christianity we are talking about here.
For Calvinists the Christian is correct here but they are speaking a different language. For Calvinists if you are not a Calvinist at death, then you were never meant to be saved. If you are a Calvinist at death then you were always meant to be saved. The confusion comes about because Calvinists do not advocate Free Will, therefore before time, God had chosen who were the chosen ones and who were reprobate.
Therefore it is reasonable for the Calvinist to state that an ex-Christian was never saved because they fell away. This of course neglects the possibility that the ex-Christian might come back to their faith - in which case they were meant to be saved, etc ad nauseum.
So if someone pulls the "once saved, always saved line" or the No True Scotsman - chances are they might be a Baptist or any protestant sect which relies on the teachings of John Calvin.