One of the arguments that I find Christians trot out immediately in conversation with someone who left the faith and became an atheist, is to claim that they were never a Christian to begin with. Christianity is based upon a personal relationship with Jesus, so it is impossible for Christians to accept that someone who had that personal relationship can now abandon it and leave the faith. The only plausible explanation is that the person was never really a Christian, that they never really knew Jesus. I have heard Christians make this claim about the most dedicated former Christians, including people who were preachers for decades. The claim is always that the person must have been faking it, or that they must have just viewed their Christianity as a dry religion rather than as a living relationship with the creator. The ironic thing is that, no one ever seems able to point to those Christians who are not real Christians while they are still Christians. It is easy to simply state that someone was never a real Christian after they have given it all up, but if they were never really a Christian, shouldn't that lack of real faith been detectable at the time?
Ironically, I find myself making the exact same statement about some people who claim to have converted from atheism to Christianity. "You weren't a real atheist, or else you never would have become a Christian. If you really understood what it means to accept that there is no evidence for deities, then you couldn't possibly become a believing Christian." I have wondered if I am just being hypocritical in my criticism of Christians for taking the easy way out in disregarding a former Christian and then doing much the same when it comes to former atheists. However, I think that my claim that someone who became a Christian was not a real atheist is fundamentally different for the reason I alluded to above. The person who claims to be an atheist and then embraces Christianity can be "outed" while they are still an atheist. If you walked me into a room full of people who claimed to be atheists and asked me to have a conversation with them to find out the people who are most likely to convert to Christianity, I believe I could do so. I believe I would be able to point out some people who would never become Christians and others who simpy don't get atheism and rationalism and therefore may one day become religious. I don't think the opposite scenario is plausible - that a Christian could do the same in a room full of Christians. I don't think anyone who met me when I was a Christian would have thought: "That guy isn't going to be a Christian in the future."
A couple of examples to illustrate my point. Take Kirk Cameron of Growing Pains fame. Everyone knows he is a Christian who had a pretty public conversion during the latter years of the show's production, and he has a fairly public profile as a Christian to this day. But he has claimed that, before he became a Christian in his late teens or early twenties he was an atheist. I don't believe that for a moment. I don't think he was ever a real atheist. I think he simply didn't think about religion or atheism, so he now categorizes himself as a former atheist just because he wasn't a Christian. I think if you had sat down with Kirk Cameron as a twenty year old and asked him why he doesn't believe there are any deities, I doubt you would have gotten a carefully thought through explanation about rationalism and lack of evidence. I doubt he would have outlined how scientific advances have, at every turn, discredited the notion of a personal God. Another example is Alister McGrath. He is a prominent Christian who also claims to be a former atheist. But, I don't think he was an atheist at all. When you listen to him in interviews, he simply mentions that he was an atheist but then became a Christian in university once he started to think for himself. (Ironic that he converted to the religion of the culture he was living in if he truly started to think for himself...notice that very few people in England or America convert to Buddhism once they become religious). An example video interview is found here:
As an aside, it is interesting to note that McGrath states that there are interesting questions that science cannot answer but which religion can, questions such as "Why are we here?" But, he never suggests why religion can answer those questions, or what the answers are. He simply comes up with something that he thinks is outside the realm of science. This is a classic example of someone not liking the answer that science provides and turning somewhere else for a more palatable, though fictitious answer.
In conclusion then, I believe it is very much possible to be a former Christian. While it might be technically possible to be a former atheist, it is very much less likely.