A couple of days ago I dropped by the blog of the late Dr. Ken Pulliam. Dr. Pulliam passed away from a heart attack, but he had apparently written a number of articles in advance, and set them to post automatically on his blog. So, in a sense, he was still publishing new material for several months after his death - which was both confusing and more than a little creepy, but that was why I was dropping by: I wanted to see if anything new had shown up since my last visit, and (on a more melancholy note) I wanted to see what had become of the blog since his passing.
By his own account, Dr. Pulliam was "born again" in 1978. He earned a Ph.D. in Theology, and took a position at an evangelical Bible college, teaching and preaching. Around 1994 (sixteen years or so later), he started having doubts. In an attempt to bolster his wavering his faith, he began a thorough study of the issues that were causing him doubts. Despite this, by 1996 he was forced to admit that he no longer believed. He was an ex-Christian; he had fallen away; he was apostate.
Now, there is a certain variety of Christian for whom that sort of story is simply unacceptable. They read it and immediately think, That can't be right. So they go to the comments section, and post something along the lines of this:
Your post name caught my attention and I just wanted to help you understand better. Keep in mind, there is no such thing as an Ex-Christian because Christians don't Fall Away. People are not "De-Converted from Evangelical Christianity", they were simply never Christians. Its hard news at first but there is a promising outcome.
This is not the true Scotsman fallacy either. There are fundamental criteria set by Christ Himself to determine who is, or who is not, a Christian. You have just not met that criteria. Essentially, you have never been saved. But actually, that is great news! Its not that it was a hit and miss, but that you never have yet been at bat yet. There is still time while you're alive.
I'm using this particular example because, to be honest, it's one of the more polite phrasings of something that is, at heart, an extremely condescending thing to say. Most of them are more along the lines of "You shouldn't call yourself an ex-Christian. It's obvious you were never a Christian at all."
This is not, to be sure, a typical Christian response - insofar as a "typical" Christian response is possible at all. It seems to be a fairly small minority of Christians who react this way. However, it is common enough to be recognizable pattern: I can think of at least three occasions where people showed up on Dr. Pulliam's blog to make this assertion, and at least two which happened elsewhere. Also, the folks who do espouse this view tend to be very insistent about it. Interestingly, I don't see this very much on atheist and agnostic blogs; it's something about defining oneself as an ex-Christian or Former Christian that seems to trigger it.
...All of which brings me, finally, to the point of this essay: if you're a Christian, and you feel that evangelism is a moral obligation for you, telling self-described ex-Christians that they were never Christian to begin with is not how you should do it.
But why not? You may be asking. I'm just trying to share the Gospel. What's wrong with that?
The problem isn't with you trying to share the Gospel. The problem is that this particular approach is arrogant, disrespectful, and fundamentally misguided. Those are not qualities that make for especially winsome or effective evangelism. I don't want to be approached this way because it's obnoxious, and you don't want to approach people this way for exactly the same reason.
Now, I realize that some of you may be scratching your heads and wondering, What's so bad about it? Well, let me unpack it for you a bit.
- I know what you were and what happened to you better than you do. The simple act of telling someone that they were never really a Christian to begin with puts you in the position of asserting that you know more about that person's life than they do. See a problem, here? No matter how politely you phrase your accusation, the underlying assumption makes you look like an asshole. This is probably not the impression you want to make when you're trying to share the Good News of Salvation through Christ.
- It's an irreducible argument. For a Christian who holds as an article of faith that true Christians cannot fall away, it's perfectly obvious that the former believer must not have been a true Christian. For someone who no longer believes in the truth of Christianity, the idea that "true Christians never fall away" just looks like something else that Christianity was wrong about. As a result, arguments on the topic tend to be nit-picky, repetitive, and unconvincing - to both sides.
- Even if you're right, you're still missing the point. Look, if someone is having doubts about their beliefs, then suggesting an alternate approach or a new viewpoint might be helpful. But, by the time someone starts defining himself as an ex-Christian, he's reached a point where Christianity as a whole no longer makes sense to him. Even if you somehow manage to convince him that he never was a Christian, so what? Christianity still doesn't make sense to him. Your efforts would be better spent trying to address the actual issues that make Christianity seem nonsensical to him, than quibbling over whether he was a Christian in the first place.
If your goal is to bring someone (back) into the flock, debating whether or not they were Christian previously is a pointless distraction.
- What kind of God would do that? If you insist that the former believer was never really a Christian, then you're also acknowledging that God was willing to let him go on believing - for at least sixteen years, in Dr. Pulliam's case - that he was saved; when in truth he was bound for Hell. This makes God look like a bit of a bastard, frankly. It doesn't seem at all compatible with the idea of God as either merciful or just. Again, probably not the impression that you really want to make - especially on behalf of the Almighty.
So here's my advice, for anyone interested in effective evangelism: don't go there. It doesn't help, and it makes you look bad. If you want to share the Gospel, do it by your actions; if you feel compelled to tell people about it, use a soft sell. Be humble and be ready to back off the moment the person you're talking to indicates disinterest. We will know you are Christians by your love, after all.
Melissa, over at Permission to Live, has a very good response to this question.
 Most Christians would be understandably wary of going up to a Christian of another denomination and telling them that they aren't really Christian. Why would you think that former believers would react any differently from current believers?
 Otherwise, they just switch churches or find a more compatible denomination. When that happens, they don't define themselves as ex-Christians, though they may think of themselves as "ex-Catholics" or "ex-Fundamentalists" or something along those lines.