How many times it has been that I've heard someone claim "I used to be a Christian, but I don't believe that any more." They have become agnostics, atheists and pagans. Conversing with them has become an apparent lost cause.
At one time they bought the false gospel of modern "evangelicalism", walked an isle, maybe been baptized and been told that their conversion rested on a prayer they prayed.
Now, having turned from from what they perceive to be the faith, they fight against it. And if you try to help them understand that they turned from a false gospel and were never converted, they curse and swear about their conversion being genuine. "I was because I know I was" is the verbal foot-stomp.
This is a plain product of modern "evangelism" which basically turns conversion into "I did this or that, therefore I can say I was converted". Never mind the plethora of verses that warn of continuance in the faith, such as 1 John (". . .they left us because they were not of us, but they left us to make it manifest that they were not of us. . ."), Philippians 1:6 ("He that has begun a good work in you will continue to perform it until the Day of Christ Jesus").
Becoming a Christian isn't a matter of saying a few words and asking Jesus to become part of your life. That's the popular view which has its roots in the error of Charles Finney. "Poor old Jesus, waiting there at your heart's door. Too bad He has to wait for you to let him in". No. It's not about doing something (praying) to get something. It isn't about taking Jesus onto a part of your life. It is about literally becoming a new creation.
Biblical regeneration is being changed from one kind of creature into another. The Bible clearly says that it is about being brought from death to life! That kind of life is eternal. It is a quality of life that never ends. It perseveres. And it perseveres because God causes it to do so. And endurance is part of the new creature.
Again, 1 John says that "whoever is born of God will not continue to practice sin. . .whoever is born of God continues to purify himself even as Christ is pure". The bottom line? If you are really converted, the best evidence that your conversion was real is that you are still being conformed to the image of Christ. The Great Betrayal of false conversion is trying to say "I once was a Christian but no more. . .I'm done with that". My friend, if you say that, the scriptures clearly say that you were never converted to begin with! Stomping and arguing to the contrary won't change that.
In the natural realm, does a butterfly go back to being a caterpillar? In the supernatural, does the sheep turn itself back into a goat? What nonsense! Of course not! But 2 Peter does talk about the dog returning to its own vomit. The dog was never transformed!
When Jesus said "You must be born again" in John 3, he was talking about a literal rebirth. A literal recreation. It is caused by a work of God (John 1:12 says we are "born not of human will but of God"). You are transformed from death to life. You are transformed from a goat into a sheep. This is an irreversible transformation. You can't reverse it, and God won't.
Have these people crossed the line of hope? I don't know. I do know I was once one of them. I bought the false gospel of "Pray and lean on what you did" (which is salvation by works). Having done that, years later I turned away and even cursed God. This was maybe 14 years ago. I distinctly remember saying (one day) "If this is how God treats His Children I don't want Him anymore!" Ahh! But I was not one of His children!
But God. Aren't those good words?
It was in 1998 that the Lord, without the intervention of human preaching, woke my conscience and showed me that I was truly lost and without hope. It was later in my struggles that I, like Paul said to Timothy, had been acting out of ignorance and unbelief. The Gospel I had turned from wasn't the real gospel but a false one. And God chose in His mercy to help me see that. This was a major breakthrough for me because I had felt so irredeemable. After all, I had cursed God! And yes, that was serious! But the Gospel I said "didn't work" was the false one I had been given. "Pray a prayer and all will be well!"
Perhaps this became the biggest stumbling block for me for many years. What God started in 1998 was only the beginning. He had to bring me through a very dark period where I had such persistent doubt, regardless of God's obvious intervention in my life. It took the Lord Himself to get me past my arguing to a point where I knew that the "gospel" that I had embraced and turned from was not the Biblical one. It had been about "me doing", not what God has done and my trust being in that alone. And that is the eternal difference. Again, I had been acting "in ignorance and disbelief"!
But there is an eternal danger. The eternal danger is hanging on to the old "I know I was a Christian and it was all false" argument. This is a sure sign of inoculation. Hopefully it won't prove terminal.
Can one get to a point of no return? Sure. Hebrews 6 warns of being enlightened and tasting but turning away. Hebrews 10 also, as well as other passages, warns of falling away once one knows the truth. And Hebrews 12 warns that the local body is to make sure that no one falls short of God's Grace. In other words, realize that there can be tares in your midst and do all you can to reach them.
And the much ignored issue of Church Discipline is, really, nothing more than an element of evangelism. The person is in a sin pattern, give them the law and the message of grace. See to it that they don't fall short of the Grace of God.
Once a Christian but not now? No such animal exists. You don't become a Christian by self-proclamation. It isn't by your own choice, regardless of what the contemporary church says.
Be sure that you aren't deceived.