The doctrine ‘once saved always saved’ is not found in the Bible.
Salvation is and always has been conditional.
The parable of the sower illustrates this.
Matthew 13:18 Hear ye therefore the parable of the sower. 19 When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side. 20 But he that received the seed into stony places, the same is he that heareth the word, and anon with joy receiveth it; 21 Yet hath he not root in himself, but dureth for a while: for when tribulation or persecution ariseth because of the word, by and by he is offended. 22 He also that received seed among the thorns is he that heareth the word; and the care of this world, and the deceitfulness of riches, choke the word, and he becometh unfruitful. 23 But he that received seed into the good ground is he that heareth the word, and understandeth it; which also beareth fruit, and bringeth forth, some an hundredfold, some sixty, some thirty.
So does the parable of the talents
Matthew 25:14 For the kingdom of heaven is as a man travelling into a far country, who called his own servants, and delivered unto them his goods. 15 And unto one he gave five talents, to another two, and to another one; to every man according to his several ability; and straightway took his journey. 16 Then he that had received the five talents went and traded with the same, and made them other five talents. 17 And likewise he that had received two, he also gained other two. 18 But he that had received one went and digged in the earth, and hid his lord’s money. 19 After a long time the lord of those servants cometh, and reckoneth with them. 20 And so he that had received five talents came and brought other five talents, saying, Lord, thou deliveredst unto me five talents: behold, I have gained beside them five talents more. 21 His lord said unto him, Well done, thou good and faithful servant: thou hast been faithful over a few things, I will make thee ruler over many things: enter thou into the joy of thy lord. 22 He also that had received two talents came and said, Lord, thou deliveredst unto me two talents: behold, I have gained two other talents beside them. 23 His lord said unto him, Well done, good and faithful servant; thou hast been faithful over a few things, I will make thee ruler over many things: enter thou into the joy of thy lord. 24 Then he which had received the one talent came and said, Lord, I knew thee that thou art an hard man, reaping where thou hast not sown, and gathering where thou hast not strawed: 25 And I was afraid, and went and hid thy talent in the earth: lo, there thou hast that is thine. 26 His lord answered and said unto him, Thou wicked and slothful servant, thou knewest that I reap where I sowed not, and gather where I have not strawed: 27 Thou oughtest therefore to have put my money to the exchangers, and then at my coming I should have received mine own with usury. 28 Take therefore the talent from him, and give it unto him which hath ten talents. 29 For unto every one that hath shall be given, and he shall have abundance: but from him that hath not shall be taken away even that which he hath. 30 And cast ye the unprofitable servant into outer darkness: there shall be weeping and gnashing of teeth.
Paul, during his ministry, didn’t count himself as saved
Philippians 3:12 Not as though I had already attained, either were already perfect: but I follow after, if that I may apprehend that for which also I am apprehended of Christ Jesus. 13 Brethren, I count not myself to have apprehended: but this one thing I do, forgetting those things which are behind, and reaching forth unto those things which are before, 14 I press toward the mark for the prize of the high calling of God in Christ Jesus.
He recognised that even he could be a castaway
1 Corinthians 9:27 But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
He says that there will be a ‘falling away’.
2 Thessalonians 2:3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
Some will ‘depart’ from the faith:
1 Timothy 4:1 Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith…
Men would arise from amongst them with false doctrine.
Acts 20:30 Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them.
Those who had received Holy Spirit gifts of supernatural powers could also fall away, and if they did reconciliation would be impossible.
They would be counted as thorns and briars to be destroyed.
Hebrews 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Spirit, 5 And have tasted the good word of God, and the powers of the world to come, 6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame. 7 For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God: 8 But that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned.
We are saved by the gospel only IF we keep in the gospel in mind.
1 Corinthians 15:1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; 2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
Some in the first century ‘made shipwreck’ of their faith and were excommunicated.
1 Timothy 1:19 Holding faith, and a good conscience; which some having put away concerning faith have made shipwreck; 20 Of whom is Hymenaeus and Alexander; whom I have delivered unto Satan, that they may learn not to blaspheme.
Peter tells us that we have to ‘give diligence’ to ‘make sure’ of our salvation.
This indicates that we can fail if we don’t ‘give diligence’ to make it sure.
2 Peter 1:10 Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall:
Peter speaks of those who, having been washed, return to wallowing in the mire.
2 Peter 2:18 For when they speak great swelling words of vanity, they allure through the lusts of the flesh, through much wantonness, those that were clean escaped from them who live in error. 19 While they promise them liberty, they themselves are the servants of corruption: for of whom a man is overcome, of the same is he brought in bondage. 20 For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. 21 For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them. 22 But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.
The writer to the Hebrews says that we won’t escape if we neglect our salvation.
Hebrews 2:1 Therefore we ought to give the more earnest heed to the things which we have heard, lest at any time we should let them slip. 2 For if the word spoken by angels was stedfast, and every transgression and disobedience received a just recompence of reward; 3 How shall we escape, if we neglect so great salvation;
Paul tells the Philippians to work out their salvation with ‘fear and trembling’.
Philippians 2:12 Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
We are saved IF we ‘endure’ to the end
Matthew 10:22… he that endureth to the end shall be saved.
The letters to the churches in Revelation show that they are far from ‘once saved always saved’.
Laodicea for example:
Revelation 3:14 And unto the angel of the church of the Laodiceans write; These things saith the Amen, the faithful and true witness, the beginning of the creation of God; 15 I know thy works, that thou art neither cold nor hot: I would thou wert cold or hot. 16 So then because thou art lukewarm, and neither cold nor hot, I will spue thee out of my mouth. 17 Because thou sayest, I am rich, and increased with goods, and have need of nothing; and knowest not that thou art wretched, and miserable, and poor, and blind, and naked: 18 I counsel thee to buy of me gold tried in the fire, that thou mayest be rich; and white raiment, that thou mayest be clothed, and that the shame of thy nakedness do not appear; and anoint thine eyes with eyesalve, that thou mayest see. 19 As many as I love, I rebuke and chasten: be zealous therefore, and repent. 20 Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me. 21 To him that overcometh will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in his throne. 22 He that hath an ear, let him hear what the Spirit saith unto the churches.
In conclusion those who believe the gospel and are baptised into Christ can lose their salvation if they fall away from the gospel.
I hope you find this helpful.