As little as I have in common with Calvinists, I find the Perseverance of the Saints to be most compelling. Salvation, once truly received, cannot be truly lost, for it is the Holy Spirit who seals us until the day of redemption as a guarantee (2 Corinthians 1:22; Ephesians 1:13; 4:30). Therefore, as Paul says, "But we are not of those who draw back to perdition, but of those who believe to the saving of the soul." (Hebrews 10:39).
But then, what of those who draw back? Whatever the Bible says, we must wrestle with the fact there are people in this world who claim, and who may even genuinely believe, they were once Christians, yet now, they do not believe. If we cannot lose salvation, and especially if we are sealed by the Holy Spirit as a guarantee until the day of redemption, how is it that some people apparently manage to break that seal and lose their salvation?
In Matthew 13:3-9, we read "Then He spoke many things to them in parables, saying: “Behold, a sower went out to sow. And as he sowed, some seed fell by the wayside; and the birds came and devoured them. Some fell on stony places, where they did not have much earth; and they immediately sprang up because they had no depth of earth. But when the sun was up they were scorched, and because they had no root they withered away. And some fell among thorns, and the thorns sprang up and choked them. But others fell on good ground and yielded a crop: some a hundredfold, some sixty, some thirty. He who has ears to hear, let him hear!”"
In verses 18-23, Jesus explains the parable: “Therefore hear the parable of the sower: When anyone hears the word of the kingdom, and does not understand it, then the wicked one comes and snatches away what was sown in his heart. This is he who received seed by the wayside. But he who received the seed on stony places, this is he who hears the word and immediately receives it with joy; yet he has no root in himself, but endures only for a while. For when tribulation or persecution arises because of the word, immediately he stumbles. Now he who received seed among the thorns is he who hears the word, and the cares of this world and the deceitfulness of riches choke the word, and he becomes unfruitful. But he who received seed on the good ground is he who hears the word and understands it, who indeed bears fruit and produces: some a hundredfold, some sixty, some thirty.”
From this, we see that people actually fall into a few categories when it comes to "the word of the Kingdom". There are those who don't understand it, and so the devil takes it from them. There are those who hear and receive it with joy, but endure only for a while, but tribulation or persecution arise, and he stumbles. There are those who hear it, but are tempted by the cares of the world and the deceitfulness of riches. Finally, there are those who hear, understand, and faithfully live out the word.
Of these 4 grounds, note how only the first kind does not seem to become a Christian. He who does not understand the word, naturally, does not receive it. He has no time for anything to take root, the enemy just snatches it all like a bird takes a seed. Now the ground is as bare as if the seed was never there.
Yet, the other two kinds of unfruitful grounds actually seem to have done some growing. It may be that their root doesn't grow down, or it may be that their seed competes with other plants that inevitably choke them out, but ultimately, something grows in them. Now, here is the question: Did the ground change, or did the seeds simply fail to thrive in the ground?
The obvious answer is that the ground itself is the problem. If the ground does not first change, the seed will never successfully grow. Thus, it is inevitable that such people will apostatise.
It could be that the "Christian" did not understand their Christian faith. This is often the case. How many Christians, when asked "why are you a Christian?", would be forced to answer "because my parents raised me Christian"? But any competent church will teach its members that they are not a Christian because someone else told them to be! As Christians, we almost have our own language! Try explaining to some random Joe on the street, "hey, you're a sinner, but if you repent, and believe Jesus is the Son of God, you'll be saved by grace through faith". There are several terms in there that require elaboration. What is sin? What is repentance? What does it mean to believe, or have faith? What does it mean that Jesus is the Son of God? What is grace? Those are 5 important questions that some people recite blindly. It's like e = mc². We can all cite it from memory, but how many of us actually understand what Einstein's famous formula actually means?
In much the same way, so many Christians recite Christian beliefs, mantras, creeds, even Bible verses, without having the first clue what any of it means. This is such a prominent problem within the Church that we can honestly say that you don't even need to apostatise explicitly in order to not be a Christian. It's not only why so many false denominations exist, but also why they are so frequently validated.
There are 4 other reasons Jesus cites between the two other bad grounds: Tribulations, persecutions, the cares of the world, and the deceit of riches. Each of these are powerful motivators for people to do all sorts of things. Although God made and sustains the world, the world itself is not God. Therefore, it is entirely possible to live life without duly acknowledging Him, and by default, we all do. In fact, if we are not blessed with parents who raise us to understand the word, the world is all we will know. The word, ultimately, disrupts that. It changes us, and in particular the way we interact with the world. In turn, it changes the way the world interacts with us.
So, what happens when the devil's birds fail to snatch our seeds? Well, naturally, he sends the sun to burn up our exposed roots. And if that fails? Well, here come the thorns. The devil will do anything and everything to tempt us away from the faith. And He can succeed if we are the right kind of ground.
For this reason, Jesus actually has some very harsh words for us: "If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell. And if your right hand causes you to sin, cut it off and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell." (Matthew 5:29-30). Now, praise be to God, He was not being literal here. We do not have to cut off our eyes or limbs because Christ was cut off from the land of the living (Isaiah 53:8). But this does reveal to us that we must do a bit of terraforming. When we receive the word, it doesn't matter what kind of ground we are, the devil is coming for us. So if seed falls by our wayside, pick it up and bury it before the birds pick it up and eat it. If it falls in stony ground, remove the stones, let those roots grow down before tribulations and persecutions burn them up. If there are thorns, uproot them; don't be fooled by riches, or the cares of the world.
But some people do not heed this. They do not take care to ensure they understand and receive the word with all readiness, nor do they prepare for the inevitable temptations that will come for them. Thus, they will always be the kind of ground they were. Now read the words of John:
"Little children, it is the last hour; and as you have heard that the Antichrist is coming, even now many antichrists have come, by which we know that it is the last hour. They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us. But you have an anointing from the Holy One, and you know all things. I have not written to you because you do not know the truth, but because you know it, and that no lie is of the truth. Who is a liar but he who denies that Jesus is the Christ? He is antichrist who denies the Father and the Son. Whoever denies the Son does not have the Father either; he who acknowledges the Son has the Father also." (1 John 2:18-23).
Verse 19 is particularly important here. "They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us."
According to this logic, ex-Christians were never truly Christians. They may have appeared to be so, as the seed which grew on bad ground may truly have grown. That is because the word is true. But because the ground was not suitable for growing, anything that grew in it would eventually fail to do so. That was always the case. They didn't become a Christian, then cease to be one, they appeared to be a Christian, or maybe just identified as a "cultural Christian". Then, as John says, it was made manifest just what kind of ground they really were.
This can actually be seen in the most vocal examples. When "ex-Christians" apostatise, they will often seek to bring other people with them. But in so doing, they often seem to be worse at it than those who never professed faith. Their arguments are sloppy and amateur. They say things about the faith that make true believers roll our eyes. They fall for, and repeat, the worst arguments unbelievers have to make.
What's more is that without explaining what changed their mind, they even make arguments they previously resisted, support that which they previously opposed, and oppose that which they previously supported. Their change of heart seems to be without reason, because it was never really a change of heart. The only thing that has really changed is their belief in God.
But what does James say? "You believe that there is one God. You do well. Even the demons believe—and tremble!" (James 2:19). To believe in God is a good thing, and it can, out of fear, cause one to do good. But faith is more than just believing. It produces what the Bible describes as "Godly sorrow", leading to repentance (2 Corinthians 7:10). It produces love, which casts out fear (1 John 4:18). Belief without such things causes trembling. In other words, any obedience is begrudged.
But get rid of that belief, you get rid of the trembling. Now, you're "free" to enslave yourself to the same thing that mastered you before. Popularity, money, sexual immorality, power, all of these temptations a Christian would shun, you now get to embrace as if it was a good thing. And the world will continue to press the lie; it is a good thing.
As Christians, we are set free from that lie, and the death that it brings. Of course, such things are too complex for unbelievers themselves. We should thus not overwhelm ex-Christians with the idea that they were never truly Christians. In so doing, what we really tell them, in their mind, is "you are not a sincere person". This is simply because they have become the wayside; too hard for a seed to even be planted. You can rest assured, the devil's birds will snatch that seed in the blink of an eye.
There is good news, however; the same lies "ex-Christians" believe and spout are the same lies as those who never professed faith. Therefore, you can win them "back" to the faith in the same way you win other unbelievers. Preach the word, live the word, pray that God will grant them repentance.