One of the issues that I, as a former Christian, often face in conversation with Christians is the notion that I could not have ever really been a Christian if I have now left the faith. A common attitude amongst Christians is that no one, once they had truly been a Christian, could leave it behind and become an atheist. I find this attitude arrogant and cowardly.
Arrogant because it assumes that Christianity is the truth, and the only truth. This is nothing new amongst Christians. Anyone who has had even an elementary discussion about religion with a Christian will almost invariably have found them to assume that they are right and you are wrong. The prevailing attitude is often one of reluctant willingness to engage in a discussion, but only with the pre-arranged understanding that they can't change their mind because they are already right. I suppose on some superficial level I can understand this attitude. If I think about something that I am almost certain about -- gravity for example -- I suppose I would have a hard time keeping an open mind in conversation with someone who didn't believe in gravity. Perhaps even with my belief of atheism I could relate. Do I think it is possible for someone who has truly understood atheism to then become religious? I think it's unlikely, but I certainly have an open mind to the possibility. I don't think that an atheist who becomes a Christian automatically and retroactively becomes someone who never truly understood atheism in the first place.
Cowardly because it is a convenient way of avoiding an inconvenient confrontation. If a Christian meets someone like myself who was a true Christian for many years and then left it behind, they have to start facing some very difficult questions. Such as: Why did you stop believing? Aren't you afraid of hell? What questions did you ask about your faith that lead you to realize it was false? Instead of facing these questions, it is easier (and less scary) to simply state that I could never have been a Christian to begin with. That solves the problem in one easy statement. Someone who never was a true Christian didn't really understand Christianity and so could not have asked some uncomfortable questions about the religion. Instead, they must have just drifted away through laziness or selfishness (the Christian assumption that all atheists are inherently selfish is a whole other topic, worth discussing).
Ultimately perhaps the issue is best resolved by examining how you define a Christian. Christians often define themselves as people who believe that Jesus is the son of God (the specific Biblical Yahweh, though of course many Christians don’t know their god by that name), that Jesus died to save them personally from sin and death, and who rose from the dead following his death. Of course, there are many other things one associates with the definition of a Christian, including changes to their life and an attempt to live their life a certain way. But ultimately I think most Christians would define their faith as a relationship. A relationship with Jesus.
So, by this definition, I think I qualify as a former Christian. I was a Christian by the very definition that Christians themselves use to define themselves: I believed that Jesus Christ was the son of God and that he died to save me from my sins. I didn't just say I believed it, I really did believe it. I also had a relationship with Jesus. Or so I thought. Now, of course I recognize that it was all in my head, and there was no relationship since a relationship with someone who no longer exists is not possible. Perhaps this is the loophole Christians will grab onto to insist that I never was a Christian: you never actually had the relationship with Jesus, otherwise you could never come to the point that you think it was all in your head. But the reality is that all relationships with Jesus are imaginary, so I fit the definition of having that relationship in the same way as anyone who has ever claimed to be a Christian. Now I no longer believe in Jesus, nor do I claim to have a relationship with him, which puts me in that most awkward category of persons that Christians must face: a former Christian.