You: I used to be a Christian but I stopped believing in God.
Christian: Ah, but the Bible says you can't lose your salvation, so if you stopped believing, you can never really have believed in the first place, can you now?
Well, this is my response to that:
Unlike some of the other claims certain Christians make, this is practically known to be false by ex-Christians. For instance, the claim that homosexual acts are sinful might be backed up by an argument and the claim that the planet is only a few thousand years old at least has the merit that yes, the world could've been created last Thursday for that matter with a load of spurious evidence and false memories, although it very probably wasn't, particularly considering that God, being perfect, is not supposed to deceive us. This, however, requires a much higher degree of scepticism on behalf of the Christian because unlike the other claims, it seems to be one of two things. Either the Christian is accusing the ex-Christian of making a false statement, possibly of lying in fact, or they are claiming their memories of being a Christian are false.
This is an extremely silly thing to do because this means the atheist doesn't just believe confidently that the Christian is incorrect, but is so close to certainty that it would be laughable to quibble about the hair's breadth of difference between the two. And yet they still do it.
In Christian theology, there is a concept known as the "perseverence of the saints". This means you cannot give up your salvation. It's a bit like being British. Provided you have at some point in your life honestly believed in God, God's goodness and you have acted on that knowledge to repent of your sins and commit yourself to Christ as fully divine and fully human Lord and Saviour - basically, what it says in the creeds - then you are, in terms of evangelical Christianity, now permanently saved, i.e. a Christian. You can't lose it again.
Clearly there are a lot of people who no longer believe, and i was one of them for something like twelve years. This was after i committed myself to Christ. I genuinely, honestly believed and i really lost my faith and stopped believing. That really happened.
I also suggest that atheists can use this fact to annoy Christians and defeat their claims, by claiming honestly still to be Christians in terms of that theology. The trouble with this is of course that once again, a Christian is being presumptious - i am suggesting that this is what you do, which isn't really my place, is it? However, i do think it might be amusing, on occasions, to be arsey and say "Yep, I'm Christian. Of course, I don't believe all that rubbish about imaginary sky beings or God being his own father and turning into a zombie who went into space, but none of that matters. All that matters is that I committed myself to Christ one weekend a quarter of a century ago when I was in an emotionally vulnerable position, then recanted when I came to my senses on Monday. Praise the non-existent Lawerd!"