In our world today, many people claim to be ex-Christians. I do not say "claim to be" as an attack on their character. Rather, although it is entirely possible to sincerely claim and believe to be an ex-Christian, this is actually as illogical as claiming to be an ex-adult. This is because Christians receive the seal of the Holy Spirit as a guarantee of their salvation (2 Corinthians 1:22; Ephesians 1:13; 4:30). No spirit, no Christianity.
As shown in the article linked above, there are many reasons people may believe they were Christians, and in some cases, even atheists might claim they are still Christians because of these reasons. But with the exception of "I was baptised as a child", most reasons for believing you're an ex-Christian fall under one category: Intellectual assent.
Of course, given that the Bible presents belief in Jesus as the sole criteria for salvation, it's easy to see why intellectual assent might convince someone they are an ex-Christian. "I believed in Jesus, I now do not believe in Jesus, therefore I am an ex-Christian". And honestly? That's absolutely fine. For sake of argument, we can usually afford to let non-Christians claim they are ex-Christians, especially given that outside of this discussion, it's usually a trivial detail. For sake of this discussion, however, we can divide apostates into two categories.
The first, of course, is the aforementioned intellectual assent. This applied to Judas, as a matter of fact. Judas had the privilege of not only meeting Jesus, but actually being chosen as a disciple. Judas walked with Jesus. Judas heard Jesus speak. Judas saw Jesus' miracles. Judas, in some cases, was actually given the ability to perform miracles with the other disciples. And as you well know, it's very hard to reject what you have seen with your own two eyes. When it comes to intellectual assent, Judas had the lion's share. Yet, it took a mere 30 shekels of silver to convince Judas to turn Jesus over to His persecutors.
Now, that is absolutely not to say that all ex-Christians are motivated by greed. Some of them are, Judas wasn't unique, and greed is a part of sin nature. Many of us, as humans, are particularly susceptible to it, and so yes, money has turned many an intellectually assenting Christian away from the faith. But whatever your reason for turning away from Christianity, even if it is as sincere as "I was raised Christian, but I never saw any evidence, so I fell away as an adult", you actually have a lot in common with Judas.
Intellectual assent is strong, but unless the heart is synchronised with the head, you are not a true Christian. Judas, walking as Jesus' disciple, was not chosen for His faith, but rather, the exact opposite. "Jesus answered them, Have not I chosen you twelve, and one of you is a devil?" (John 6:70). Judas' head and Judas' heart were misaligned. Intellectually, he knew Jesus was Lord, but spiritually, he was more inclined to follow Satan.
Though a bad analogy, you could compare this to your dietary preferences. Intellectually, you may be very well aware of what foods are good for you and what foods make a very unhealthy diet. I can certainly attest to this. I know my diet is unhealthy, but even though I'm physically unfit, I'm not changing much.
But there is one particular change I have made to my diet. For pretty much my entire life, I have absolutely hated water. I'd drink milk, I'd drink squash, and more recently, I'd have a large supply of fizzy, usually sugary drinks by my bed at all times. If I drank water at all, it would be during a session at the gym, where of course the sugary drinks would threaten to come back up and bring my lunch with them. However, recently, a disease hit me like a truck. One of the symptoms? Constant dehydration. Both during my stay in hospital, and after I was discharged, I was never more grateful for even the smallest gulp. My head always understood the importance of water, but it wasn't until I knew what it was truly like to need water that my perspective truly changed. Now, although I will occasionally drink other things, my primary source of hydration is the one drink I never thought I'd willingly choose.
In much the same way, it is entirely possible to believe in Jesus, yet have no true relationship with Him. Past intellectual assent may make a person believe they are ex-Christians, but the absence of any real relationship with Christ means they actually weren't. Of course, they may still believe they had even that, but here's the key: A relationship with God changes the very soul.
Which brings us to our second category of apostate. These are much more aligned with Satan than those who have simply lost intellectual assent, because they have not lost intellectual assent. Rather, for lack of better term, this is a "lover's tiff", so to speak. The relationship still exists, it's just broken, for some reason or another. It may be a big reason. Perhaps the loss of a child. That's an understandable reason to turn your back on God. Not a good reason, but an understandable one. Other reasons are not so great, such as studying the Bible and finding out God doesn't agree with your favorite sin (or knowing the whole time, but finally giving in and abandoning God so you can do it anyway).
Although I have never fully apostatised, I can certainly sympathise with the latter. After a rather severe betrayal, and by a so-called Christian I had thought God sent as an answer to prayer, no less, I had severe mental struggles. I still believed in God, and I even continued studying the Bible, but after telling God I was angry with Him for allowing this to happen, I went my own way. I used sin as a comfort blanket. The way I was living, while not completely atheistic, was so close that I very well could have apostatised. This particular experience is why I am particularly confident in my belief in the Perseverance of the Saints. I couldn't leave God if I tried.
Satan, on the other hand, did. Being far closer to God than any man, still in the flesh, could ever be, Satan decided that his pride was more important to him than even his very soul. He lead a rebellion against God, but of course failed. To this very day, Satan knows two things: 1. what he had, 2. where he's going. Now, imagine knowing you were the "seal of perfection" (Ezekiel 28:12), only to be puffed up in pride, lose it all, and know that your days are short. It is entirely possible to fully believe in God, and yet not worship Him, even having once been very close with Him.
For many ex-Christians, it really is a case of the head. They used to believe, they now do not believe. But for many others, the encounter with God was so real, it changed the soul. But that change was not enough. Like Satan, the ex-Christian knows there is a God, not based on evidence, but based on experience, and experience is nigh impossible to shut out.
But here is the difference between the latter category and Satan: Christ did not die for Satan. No matter what happens, Satan cannot be saved. His sins were never accounted for. They will be repaid eternally in Hell, even if he was to turn around tomorrow and beg God for another chance. But as long as a human being lives, he can be redeemed.
If you claim to be an ex-Christian, you should know the Gospel. Maybe you don't. Maybe, you fall so deep into the intellectual assent category that it wasn't even Christianity you were assenting to. Put simple, the Gospel is this: God is your Creator, perfect and Holy in His ways, yet by your sin, you are at enmity with Him, and so justice dictates that you must perish. But in His love, He instead became a man, lived a sinless life, and died for sins. Your sins. By confessing Him as Lord and believing He rose from the dead, you will be reconciled to God and receive eternal life.
If you're an ex-Christian, whichever category you fall under, it's not too late. You're alive, you have time to repent. But don't gamble with how much time. Your soul is precious, both to God and to you, and you have a place in Heaven. Do what neither Judas nor Satan would do: Swallow your pride, and humble yourself before God. Then you will be a true Christian, which will bring with it rewards as eternal as your faith.