This is the question people actually lose sleep over. Someone prayed, believed, served, and then walked away and never came back. Were they saved and then lost? Or were they never saved? Or are they still saved and simply wandering?
The New Testament makes it hard by doing two things at once. It gives some of the strongest in any literature: no one will snatch them out of my hand, nothing can separate us from the love of God, he who began a good work will complete it. And it gives warnings just as strong: watch that you do not fall, if you do not continue you will be cut off, and the passage in Hebrews 6 that has troubled readers for nineteen centuries.
Every position below affirms both sets of texts. The difference is in what work the warnings are doing, and in what happened to the person who left.
Hebrews 6:4-8
4For concerning those who were once enlightened and tasted of the heavenly gift, and were made partakers of the Holy Spirit, 5and tasted the good word of God and the powers of the age to come, 6and then fell away, it is impossible to renew them again to repentance; seeing they crucify the Son of God for themselves again, and put him to open shame. 7For the land which has drunk the rain that comes often on it and produces a crop suitable for them for whose sake it is also tilled, receives blessing from God; 8but if it bears thorns and thistles, it is rejected and near being cursed, whose end is to be burned.
All the views agree on more than the argument implies. Salvation is by and not by achievement. God is faithful and does not abandon people. The warnings in Scripture are real and are meant to be heeded rather than explained away. Presumption is dangerous and so is despair. And a life that shows no fruit at all over time is a serious matter in every tradition, whatever account each gives of it. Nobody here teaches that you can be indifferent to how you live and count on a past decision.
Those God saves, he keeps
Everyone truly born again will persevere to the end, because God preserves them. Those who finally fall away show that their faith was never the real thing.
What it claims
Salvation is God's work from beginning to end, and a work God begins he finishes. If a person could finally be lost, then the decisive factor would be the person's grip on God rather than God's grip on the person, and assurance would be impossible. John makes the diagnostic explicit: they went out from us because they were not of us. Departure does not prove that grace failed; it reveals what was there. This is not once saved always safe, and its defenders resist that slogan. The claim is that genuine faith perseveres, which means continuing is the evidence rather than the exception.
Who holds it
Reformed and churches, most Baptists, and much of evangelicalism. The of Dort and the Westminster define it. The P in the TULIP acronym. Augustine taught a version of it, holding that God grants perseverance itself as a gift.
The texts it stands on
John 10:28-29 They will never perish, and no one will snatch them out of my hand. Stated twice, of the Son's hand and the Father's.
Philippians 1:6 He who began a good work in you will bring it to completion. The subject of the sentence is God.
1 John 2:19 They went out from us, but they were not of us. John treats departure as revealing rather than as losing.
Strongest case
It grounds assurance in God's character rather than in the believer's consistency, which is the only place assurance can survive a bad year. It also handles the strongest preservation texts without qualification, and it has a genuine answer for rather than being embarrassed by it.
Hardest question
The answer for apostasy is also its weak point, because it can be applied after the fact to anyone at all, which makes it unfalsifiable and can leave sincere people wondering whether their own faith is the real kind. And the warning passages, on this account, warn about a danger the elect are not actually in, which critics say drains them of force. Hebrews 6 describes people who were enlightened, tasted the heavenly gift, and shared in the Holy Spirit, which is a great deal to say about someone who was never converted.
Holding the map
The camps are further apart in theory than in practice. Every one of them tells a struggling believer the same thing: come back, keep going, do not presume, do not despair. What differs is the account given afterward of what happened.
One thing worth watching is that both sides can produce the same pastoral damage from opposite directions. A hard doctrine of perseverance can leave someone auditing their own conversion for evidence. A hard doctrine of losable grace can leave someone counting sins. Neither is what the texts are for. The warnings in Scripture are always attached to a remedy, and the assurances are always attached to Christ rather than to a moment in your past.
Questions that press on this one
None of these stands on its own. What you conclude here narrows what you can say there.
