Let's take the case of a hypothetical Christian, Jane, who accepts this doctrine. She believes herself to be a "true Christian." She sincerely believes that Jesus is Lord, and that God raised him from the dead, and that she accepted him as the savior of her life. But here's the problem: there is some possibility—even if it's only a small one—that at some point in the remainder of Jane's lifetime she will reject Christianity and either become nonreligious or an adherent of some other religion.
Now, Jane could deny this as a possibility. But life often presents us with unexpected twists and turns, so to rule out such an outcome would be narrow-minded and naïve. So if Jane accepts the possibility that she could reject Christianity in the future, she must accept the possibility that she is not currently a Christian—in spite of her sincere belief! She would likely find this conclusion patently absurd, and I find the conclusion that I was not a Christian absurd for more or less the same reason.
Now Christians can (hopefully) see how their assertion looks from my perspective. But some of them might say, "I know I'm a Christian because my faith resulted in good works. You unbelievers didn't bear spiritual fruit, so your Christianity was false." But this has two problems. First, the Bible doesn't necessarily require these fruits for salvation. Romans 10:9 states, "That if you confess with your mouth that Jesus is Lord, and believe in your heart that God raised Him from the dead, you will be saved." All this verse requires is a sincere confession of belief in Christ's deity and resurrection—no ifs, ands, or buts.
Can one find verses that say there's more to being a Christian than sincere belief? Certainly. But what that shows me is that the Bible is at worst contradictory, and at best startlingly unclear, regarding what is by far the single most important element of Christianity. If we're to be punished or rewarded eternally according to a certain set of criteria, isn't it strange that the Bible doesn't just lay out all in one place (or better yet, in many places) exactly what those criteria are? It seems rather cruel of a supposedly omnibenevolent God to leave the matter even slightly ambiguous.
Second, let's assume the objector is correct. In fact, let's raise the stakes even higher. In Luke 9:23 Jesus says, "If anyone desires to come after Me, let him pick up his cross daily, and follow Me." If we interpret this as a requirement for "true Christianity," then almost the entire church consists of fake Christians. Yet I can point to ex-Christians who met even this ridiculously high standard. Ex-preachers like John Loftus and Dan Barker once dedicated their lives completely to Christ and produced plenty of so-called spiritual fruit. If they weren't "true Christians," then no one is.
Those who claim that ex-Christians weren't Christians at all probably haven't had any experience with them. My advice to them? Read the testimonies linked in the above paragraph. Read the stories from regular people on the ex-christian.net forums. Read Ebonmuse's excellent essay "Into the Clear Air," which describes the agonizing process that Christians go through when they leave Christianity. And read blogger Luke Muelhauser's detailed account of the richness of his former relationship with God. If they've read all that and still believe that ex-Christians must have been insincere, they are only revealing their dogmatic adherence to a view that bears no resemblance to reality.