While, as others have mentioned, this is a transparently self-serving "No True Scotsman," it is also pretty self-consistent: A "True Christian" never falls away, you fell away, therefore you were obviously never a "True Christian." Simply yelling about how stupid that is isn't an effective argument.
My favorite response is to provisionally accept their logic, and offer my own syllogism:
1. Lot of people believe that they are "True Christians."
2. Some of those people later stop believing. (Implying that they were not "True Christians.")
Therefore: It is possible to believe that you are a "True Christian" but not actually be one.
This, of course, means that the person accusing ex-Christians can't 100% guarantee that he's really saved, either. The comeback, of course, is for him to argue that no, being really saved is unmistakable, but that's not helpful, since a lot of the ex-Christians also thought their experiences were unmistakable. The discussion descends into the ex-Christian explaining the depth of his "spiritual experiences" and explaining that they ultimately didn't matter, and the "once-saved-always-saved" proponent countering by saying that he's had the same experiences, but they're more real because shut up.
I prefer this line of argument because it shifts the focus from accusing the apostate of lying to demanding that the accuser prove he is telling the truth.
