During grad school, I was working in a research lab and chatting with some of my fellow students/co-workers. I don’t remember what I was saying, but something having to do with church when one of my colleagues proudly said, “Yeah, I did the youth group thing, but I don’t do the Christian thing anymore.” He went on to give sage advice about what Christians are and how he’s outgrown them. I told him I thought he was mistaken, but left it at that if he didn’t want to pursue the topic, which he didn’t. What I wanted to tell him is, “You may have hung out with Christians and may have considered belief in God, but you were never a Christian.”
I have heard this said a few times, but have, as in this case, usually not responded to it directly. I can here. I must start by saying most emphatically that there is no such thing as a “former Christian.” Some Christians might debate this, but I think the Bible is pretty clear that once God “picks” you for salvation, He doesn’t change His mind.
How do I know? Because the Bible tells us that Christianity is something you either have or do not. It is something you have gained or not. It is like a ray in math. Christianity has an origin and a direction, but no end point. All of life’s bumps and even death is just an event along the ray, but nothing will ever terminate that ray of faith.
Evidence of Salvation
Perseverance is defined by scripture to be the evidence of salvation. Job 17:9 says, “Yet the righteous will hold to his way, And he who has clean hands will be stronger and stronger.” Proverbs 4:18, “But the path of the just is like the shining sun, That shines ever brighter unto the perfect day.” John 8:31, “Then Jesus said to those Jews who believed Him, ‘If you abide in My word, you are My disciples indeed.’” Colossians 1:21-23, “And you, who were alienated and enemies in your mind by wicked works, yet now He has reconciled in the body of His flesh through death, to present you holy, and blameless, and above reproach in His sight—-if indeed you continue in the faith, grounded and steadfast, and are not moved away from the hope of the gospel which you heard, which was preached to every creature under heaven, of which I, Paul, became a minister.”
Powered by God
This perseverance is not something we provide, but God provides. If you are a child of God, you will persevere because He upholds you, not because you want to follow him. Psalm 37:24: “Though he fall, he shall not be utterly cast down; For the Lord upholds him with His hand.” Philippians 1:6, “being confident of this very thing, that He who has begun a good work in you will complete it until the day of Jesus Christ.” John 10:28, “And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand.” Jeremiah 32:40, “And I will make an everlasting covenant with them, that I will not turn away from doing them good; but I will put My fear in their hearts so that they will not depart from Me.”
Lack of Endurance
Finally, the lack of salvation that lasts a life time is a sign of damnation. Christians will one day be resurrected to join with God when the universe itself is finally reborn. Those who never knew God or who courted God and did not endure will spend eternity excluded from heaven. Hebrews 6:4-6, “For it is impossible for those who were once enlightened [learn of God], and have tasted the heavenly gift [tried out Christianity], and have become partakers [acquainted rather than friends] of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.” John 15:6, “If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned.” Romans 11:22, “Therefore considered the goodness and severity of God: on those who fell, severity; but toward you [true believers], goodness, if you continue in His goodness. Otherwise you also will be cut off.”
Good News
The good news is that it is never too late to come to Christ again. I added the additional notes regarding translation in Hebrews 6:4-6 above because that passage is often misused to say that a person that tries Christianity, but leaves, cannot come back. However, that’s not what the passage indicates at all. It says that if you learn all there is to learn about God and know his salvation is right, but then reject it as not enough, there’s nothing left to convince you and you are without hope. However, anyone who does not know God’s own saving grace still has time to take hold of it as long as you still have a breath to breathe.
All in all, there are no former Christians, but there are many who pretend to be. Once you become a believer, your faith will be preserved by the Holy Spirit and that is assurance indeed in a world that is so messed up. This isn’t a free ride, but even the worst Christian is better off than the best otherwise.
Cheers.

Sheesh. How do you know how God will tell the story? You're aesthete but you need to take an art appreciation course. When you encounter an unfinished painting, you suggest coloring in the rest as if it were a paint by number project. After finishing the first chapter of a novel, you close the book and tell your friends you know the ending. Be patient! The artist who's work you love only makes perfect decisions, past, present and future.
I'm not certain you read my entire post. The next to last paragraph states, "The good news is that it is never too late to come to Christ again." You are absolutely correct. I don't know what God is planning in the lives of others and I don't know who will be saved before the end. It's not my place to know for that matter. However, I am pointing out that anyone who wants to make the claim that he used to be a Christian, was not. There are no "used-to-beens" when it comes to Christianity. There are only "are-now-and-forevermores" and "will-someday-be-and-forevermores." The invitation to come to Christ remains open to all and can be accepted by anyone at any time. Come!
Stimulating post. I'd like to offer some critiques.
First, judging whether others are BrandX (e.g., "Christian") seems like the opposite of ways to work with/help people. Maybe it's more effective to start a dialog and ask questions. For me, being a Christian is not important (in fact the opposite is true). But would I enjoy a conversation with someone about it? Possibly, but not if her first words were "You're not __." My 2c.
"Christianity is something you either have or do not." I like this because a) it's extreme and b) it's all or nothing, which is symptomatic of all extremest religions. A good example, actually. It turns off conversation (actually drives it away).
I also like it because it's an insight about how people are inculcated with the thinking. If you substitute "permanent disabling virus" for "Christianity" (or "religion") you'll have a rational assessment of the state of affairs. Our minds are such that we fall prey to memes like this, sadly. For example, "Herpes is something you either have or do not." This is inaccurate because some people actually do get better and fall out of religion. Awareness can't be undone.
Thanks!
If you didn't want a discussion with someone who starts the conversation by saying, "You're not a Christian," why did you start one by posting a comment here? You've initiated just such a conversation. You see, a thought provoking challenge can be quite an excellent conversation starter. If this extreme thinking drives away conversation, as you have suggested, why has it spawned this conversation? In some cases, it actually causes conversation. Sometimes, it hits a nerve.
As for the rest, assuming I've read your comment correctly, you've placed quite a bit of faith in psychology and biology and education. You are free to place your faith in whatever form of deity you choose (and everyone believes in something greater than himself). This free will is a basic tenet of Christianity. However, if this is what you believe in, I disagree with your choice.
If you wish to discuss Christianity in particular, rather than just lumping my beliefs together with "extremist religions" in general, I would be happy to discuss the details and hear more about what you believe.
Cheers.