You'll find a lot of atheists claim they used to be Christians. However, this is impossible. Imagine, for a moment, a child tells you they used to be an adult, but they became a child again, and you should too. What would you think? You'd laugh. A child can become an adult, but once they've done that, they can't become a child again. In the same way, an atheist can become a Christian, but a Christian cannot become an atheist.
Contrary to the popular narrative, false conversions are a real thing. In fact, they are commonplace. As the saying goes, sitting in a church doesn't make you a Christian any more than sitting in a stable makes you a horse. Something actually has to happen before you become a real Christian. That something is an encounter with the Holy Spirit. The Holy Spirit seals us (Ephesians 4:30) until the day of redemption. Therefore, real Christians can no more cease to be Christians than an adult can cease to be an adult. In fact, John tells us this in 1 John 2:19, where he says “They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.”
In other words, if you're a Christian, you will keep being a Christian. If you "stop" being a Christian, you stopped so that everyone would know that you're not a Christian.
"But I was a Christian!" the atheist insists. Even knowing that someone who is capable of leaving the Christian faith is, by Biblical definition, not a Christian, there was a time in their lives when most of them genuinely believed they were Christians. And that's totally understandable, because there are several reasons they might have believed they once were Christians. Let's look at a few of these.
I was baptised as a baby.
I mentioned this one first because of an encounter I once had with an atheist who insisted she was still a Christian because of this. Many churches, including heretical ones, practice infant baptism, and many people believe this is what makes someone a Christian. Yet this practice is not found in the Bible. You won't find a single instance of infant baptism in the Bible. In fact, when baptism is mentioned in the Bible, it is done after confession of belief. In fact, it is strongly indicated that confession of belief is required for a baptism to be baptised (Acts 8:36-37).
So there are two problems here. The first is that, since you cannot be unbaptised as a child, even an atheist can still claim to be a Christian, which is insane, thus invalidating this as a criteria to be a Christian. Second, just as one does not need to be baptised to be a Christian, so also can one be baptised and not be a Christian. Therefore, even if you were baptised as a child, that is not sufficient to have been a Christian, and therefore you cannot claim to be an ex-Christian.
I believed in God.
Which god? A Muslim believes in a false god, yet Muslims are not Christians. The anti-Christ will believe in the real God, yet he will also never be a Christian. As James points out in His epistle, even demons believe in God and shudder, yet this is not sufficient to be a Christian. Intellectual assent is not faith. Thus, it is not sufficient to have been called a Christian, and is therefore insufficient to be called an ex-Christian.
I believed in Jesus
Again, which Jesus? 2 Corinthians 11:4 tells us it is actually possible to accept "another Jesus". An analogy I often give when distinguishing between Christianity and various pseudo-Christian denominations is this: Imagine, for a moment, I claimed to be Jesus. I walk around, do some magic tricks and preach some rather silly things. Would my followers be Christians? You could argue that they would be, but they would still be completely distinct from the Christianity that the real Jesus founded. Classifying my followers as Christians simply diminishes the meaning of the term "Christian", rendering it meaningless. The same is true for false Jesuses.
Just as there are many "gods", there are many Jesuses. The Jesus I worship is different from the Jesus the Mormons worship. The question, therefore, is which Jesus did you believe in? Aside from the fact we mentioned above (i.e. faith is not intellectual assent), you need a relationship with the real Jesus in order to be classed as Christian. Since you lacked that, you weren't a real Christian, and therefore aren't an ex-Christian.
Objection: What about Judas?
Answer: An excellent question! Judas, indisputably, had a relationship with the real Jesus, and of course was hand chosen by Jesus as a disciple. The question is, was Judas ever a real Christian? Setting aside the fact that Judas didn't actually survive long enough to receive the full Gospel, we also need to consider that Judas was picked for a reason: He did not have the heart of a disciple. His condemnation was written about long before He was even born (John 17:12) and Jesus knew the whole time that he was not a true disciple (John 6:70). Judas never "fell away" because he was not following Jesus honestly. He had ulterior motives. In the same way, it's entirely possible for a so-called ex-Christian to have followed the true Jesus, yet not have done so with a true heart, even if their motives were less sinister than Judas'.
I went to church
You may remember above how I spoke about stables and horses. A location is not the same as having faith. I actually remember one time in church when a strange, presumably drunk man stood up and made obscene gestures at the preacher. Was he a Christian? I can hope that he eventually found/finds Jesus, but just attending church is not enough to make you a Christian. We also spoke about Judas above. He followed Jesus everywhere, but again, even with his prominent leadership role, Jesus referred to him as a devil. If there can be a devil serving a leadership role within the Church, surely it's conceivable that even a sincere person sitting in the pews could be a false convert. Therefore, the fact that you went to church in the past is no more sufficient to prove you are an ex-Christian than the fact someone currently goes to church is evidence that they are currently a Christian.
I was raised Christian
Even if we assume both parents are Bible believing Christians and sought to instil that mindset into their children, the unfortunate fact is that faith is not transferable. No one automatically takes on the faith of their parents. Even if you take on many of the beliefs of your parents, that's still not the same as being a Christian. My cousin, for example, once said she likes a lot of Christian morals, but she is extremely hostile towards any attempt to even speak of the faith around her. The fact that you were raised Christian is not sufficient to prove you are an ex-Christian.
It is entirely possible for an atheist to have, at one point, believed they were Christians, and for them to have left that faith behind them. However, it is not possible for a true Christian to leave the Christian faith. Therefore, no matter how sincerely an atheist may believe they are ex-Christians, they simply cannot be.
Interestingly, I find many so-called ex-Christians have serious flaws in their theological understanding. Of all the people I've ever met who claimed to be ex-Christians, I cannot remember a single example of one who can even explain the Gospel, which of course is the most basic and fundamental message of the Christian faith. While I am not willing to lay this down as a rule, in my own personal experience, ex-Christians are generally less knowledgeable about the Christian faith than those who never claimed faith to begin with.
Whatever the reason someone has for believing they are an ex-Christian, the fact is it is a contradiction in terms. If you have faith, the Lord is strong enough to maintain it for you. The Lord will not suffer the righteous to be moved (Psalm 55:22). Therefore, rather than encourage ex-Christians to come back to the faith, I encourage them to come to it in the first place. An encounter with the Holy Spirit will feel quite different than the first time around.