What is the biblical basis for the doctrine of Eternal Security?

Either God saves, or doesn't save. Saul was abandoned completely because he did not repent and went through one thing or the other, half repentance, blaming the people, lying about why he kept the cattle and further wanted to keep up appearances in front of the elders. But he never sought forgiveness when he made his blunder (the blunder of not keeping any spoils), and did not repent.

If you add the "human element" to salvation, then there's no salvation there. It is you trying your best to be good enough for God.

The perfect view of salvation is in 2 stories:

1. The thief on the cross.
2. Cornelius the Roman.

Let's observe. We start with the thief. (Luke 23:39-43)

Jesus Christ said to repent and believe. (Mark 1:15, John 6:29, Acts 26:20) Repent: The thief recognised that he is a sinner, and that he is justly being punished. He had heard of Christ and he knew of His fame. He then chose to turn to Christ. Believe: He turned to Christ in his sinful state. Jesus never gave him a hard time. In that instant, he was saved. He simply repented (recognised his sinfulness, changed his mind about how to get to God, realising that he can't do it by himself) and believed (turned to Christ in faith believing that Christ would do the rest i.e, the saving).

Cornelius (Acts 10:1) A good man by all means; caring, a sharer and a man of good works. But all that goodness wasn't sufficient. God instructed Peter to go and tell Cornelius about Christ. And when Peter spoke of Christ, Cornelius believed and was saved together with his whole house. He repented (recognised his sinful nature - this after Peter spoke to him, and all his good works were good but insufficient for salvation) and believed (turned to Christ in faith believing Christ would do the rest).

Once you are saved, you are SAVED! Sanctification may take a lifetime, and that is the work of the Holy Spirit who dwells in/with you. Salvation on the other hand, is instant and is permanent!! Praise Jesus.

Remember, salvation gets you to Heaven. Works give you your reward (your crown) in Heaven.

There is no question that all Christians are eternally secure. The root argument for this doctrine is ‘predestination’.

If one looks at individual Bible verses on other subjects, pulls them out of context and then tries to conclude eternal security ‘yes’, or ‘no’, one never really finds any certainty. On the other hand if one just looks at the doctrine which directly speaks to the subject all confusion vanishes instantly. There is no doubt. It is as certain as the doctrine of God existing.

For example, in Romans 8:30 we see the full certainty of salvation from beginning to end. From calling to glorification in heaven:

And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified. (Rom 8:30, ESV)

Now predestination means God chose a person ‘for salvation’ (not salvation and then destruction). He did this ‘before the foundations of the world’:

even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us (Eph 1:4–5, ESV)

The doctrine of predestination means specific ‘individuals’ are called and that God knows them by name, calling each one. In fact every Christian are called ‘the elect.’ ‘The elect’ is not some mysterious concept but a title for any Christian.

So there is some common sense to be had here. If God chose you before your were born and called you ‘elect’ this is a ‘title of blessing.’ If God knowing you before you were born and knowing everything you will do in your life decided to given you a title that implies ‘great blessing’, God would be very much an evil liar if you ended up in hell, which is impossible. Therefore there is only one rational conclusion. All who are elect, that is Christians, are blessed, from the standpoint of eternity and nothing in time can ever possibly alter that.

Eternal security is pretty central to understanding the gospel. Those who reject the scriptures concerning the doctrine of election can still be among the elect but it can only cause much confusion in understanding the various scriptures that discuss the subject.

EDIT: This answer was initially given on 27 February 2021 to a different question - "What is the biblical basis for OSAS (once saved always saved)? How is this doctrine backed up with Scripture? How do they commonly defend their interpretations against rebuttals from critics?" That question was removed and answers to it transferred into this old question. Please read my answer in light of that, otherwise it might appear that I'm criticising this old question, when I am not.

Although this question might appear to be seeking a biblical basis for the complete assurance of salvation a believer might have, it is problematic. First problem, the “Once Saved Always Saved” label is one of derision, used by those who believe their salvation can be ‘lost’ against those who believe it cannot be ‘lost’. Second problem, the question is too vast in its range, if it expects a biblical basis to be detailed, plus deal with rebuttals from critics, without even stating what those rebuttals might be. Pardon me for stating this, but the question is a bit sloppy. However, as the request for a biblical basis for claiming salvation cannot be ‘lost’ is reasonable, this answer will deal with that.

The biblical principle that might be a good starting point is that God cannot lie. When God promises something, that promise will be fulfilled. Immediately, someone might counter, “Ah but conditional promises mean that God is not obliged to fulfil his promise if the other party fails to keep their part of the ‘deal’. Well, then we need to sort out if the promise of salvation is conditional, indeed, is it even a ‘deal’?

There is no question in Christianity but that salvation is not dished out indiscriminately by God. There are those who will be saved, and those who will not. However, that is not the question here. Nobody is wanting to know what has to happen before God grants salvation to an individual. This question seems to assume (though is not clear on this point) that salvation HAS been granted. “Once saved” implies that salvation has been granted.

In that case, to say such a gift could then be withdrawn by God would require biblical statements to show that the gift was not actually a gift but something only partially granted, or that had to be earned, or was ‘on loan’ until a deadline had been passed when it was no longer ‘on loan’ because the person had proved worthy of the gift being no longer conditional.

That is the nub of the issue. Would God hold out salvation like a carrot on a stick, in front of anyone, luring them on to keep going until they passed the finishing line, and THEN they would get the carrot? What kind of a God are we dealing with in that case?

There are many Bible texts that show that the Holy Spirit who began a good (i.e. saving) work in the believer, will see it through to completion on the day of the Lord, but I’m not even going to start citing them because that will only invite text-trading. The principles that are biblical need to be sorted out with regard to God’s gift of salvation. Is it a gift, or is it on loan, or must it be earned, or is it a carrot on a stick?

The biblical basis for such assurance with the gift of salvation that a Christian is secure in that assurance is that it is a gift. To offer to pay for, or even to contribute towards a gift would be to insult the giver. Either God gives the believer the gift of salvation, or he does not. When that gift is received then becomes the next principle to establish.

Jesus spoke in the present tense when assuring his disciples that they had passed over from death to life; that life eternal had already begun. He told them that long before any of them would die physically. Spiritually, they had been born anew of the Spirit at a certain point in time when their belief was shown in saving faith. Those who have become aware of that newness of spiritual life rest joyfully in the gift of salvation, not taking it for granted, but being transformed by grace till the day they die physically. That is the inward witness of the Holy Spirit in them. However, they do not have to convince others that they are now saved to eternity. They see many who have no such assurance of salvation, working hard all the time to eventually merit it, and they weep for them. All they can do is show by a transformed life that they have been gifted salvation and that it is utterly precious to them. If that isn’t good enough to persuade others that when God gives a gift, it’s given, text-trading won’t do it. No point in even going down that road, so I’ll end my answer here.

Obviously there are much longer answers, but I wanted to share the one that speaks the most to me (emphasis is mine):

2 Timothy 1:9-12 (NIV)

⁹  He has saved us and called us to a holy life—not because of anything we have done but because of his own purpose and grace. This grace was given us in Christ Jesus before the beginning of time, ¹⁰  but it has now been revealed through the appearing of our Savior, Christ Jesus, who has destroyed death and has brought life and immortality to light through the gospel. ¹¹  And of this gospel I was appointed a herald and an apostle and a teacher. ¹²  That is why I am suffering as I am. Yet this is no cause for shame, because I know whom I have believed, and am convinced that he is able to guard what I have entrusted to him until that day.

Here's another good one from Hebrews.

Hebrews 7:25 (NIV) Therefore he is able to save completely those who come to God through him, because he always lives to intercede for them.

And from one of my favorite passages in scripture (read the whole context, it's great!)

I Peter 1:5 (NIV) who through faith are shielded by God’s power until the coming of the salvation that is ready to be revealed in the last time.

The following passages speak of us being 'sealed' (i.e. a completed transaction, we now belong to God). Or to quote from Baker's Evangelical Dictionary of Biblical Theology: "A seal, in biblical times as today, is used to guarantee security or indicate ownership" (source).

Ephesians 1:14 (NIV) who is the down payment of our inheritance, until the redemption of God’s own possession, to the praise of his glory.

Ephesians 4:30 (NIV) And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.

2 Corinthians 1:22 (NIV) who also sealed us and gave us the Spirit in our hearts as a down payment.

This page is a great resource as well: http://bible.org/seriespage/assurance-eternal-security

In ancient days when an agreement was enacted between two parties in which one or both make promises under oath to perform or refrain from certain actions, such an agreement is translated to us in Scripture as a covenant. I have pulled a few quotes from this article and its entirety is well worth the reading.

An example of such a covenant between two men is the covenant between Abraham and Abimelech in Genesis 21:22-34 wherein conflict over a water source was resolved. This covenant regulates man's relationship with man on common ground. In such a covenant both parties bind themselves to promises to either act or refrain from acting in specific ways and penalties are bound within the agreement for oath-breaking while peace is the outcome of faithfulness:

The fundamental difference between covenants and other agreements is the relationship established between the covenant makers. Each party made specific promises and could expect certain benefits (and penalties, if the promises were broken) based on the terms of the covenant. But this relationship went far beyond legal concepts. Covenanted parties viewed each other as friends who were bound together permanently. Abraham's covenant with Abimelech allowed these two very different men to live peaceably in the same area (Gen. 21:34).

In the ratification of such a covenant livestock or other goods are often put on deposit to be lost or gained in the case of the covenant being broken.

An example of a covenant between God and man is that of the law given on mount Sinai and recorded over chapters 17-24 of Exodus. Essentially (and extremely simplified) God has covenanted with those He has rescued from slavery in Egypt to protect and bless them in the land they are going into based upon obedience to His word (summarized and codified in the 10 commandments) and to judge disobedience. The people agreed and a covenant was entered:

Now therefore, if ye will obey my voice indeed, and keep my covenant, then ye shall be a peculiar treasure unto me above all people: for all the earth is mine: And ye shall be unto me a kingdom of priests, and an holy nation. These are the words which thou shalt speak unto the children of Israel. And Moses came and called for the elders of the people, and laid before their faces all these words which the LORD commanded him. And all the people answered together, and said, All that the LORD hath spoken we will do. And Moses returned the words of the people unto the LORD. - Exodus 19:5-8

In the ratification of this covenant blood is sprinkled upon the altar (representing God) and on the people. The implication is loss of life for the one who breaks the covenant. An integral part of this covenant is the sacrificial system by which men, who cannot keep the law perfectly, may avail themselves of God's mercy and continued favor through the substitution of an animal's life for their own.

There is another covenant made between God and man and this is the covenant between God and Abram related to us in Genesis 15. In this scene God has made a promise to Abram that his children will be without number (as the stars) and that he and his children will inherit the land. When Abram asks God how he can be sure (read that, Abram asks God for a covenant) God tells him to gather animals, cut them in half, and lay them out in two rows. Normally both parties would pass between these pieces in what we refer to as "cutting a covenant" and this would signify agreement that, if the oaths made were not fulfilled, whatever happened to the animals should be done to the oath-breaker.

Strikingly, and foundationally for the doctrine of eternal security, two representations of God pass between the animal pieces:

And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces. In the same day the LORD made a covenant with Abram - Genesis 15:17-18a

Abram and his children are involved only as the object of a covenant which God has made with himself, therefore, if the promise is broken it can only be broken by God. Additionally God has agreed with Himself by passing through the pieces that the same should be done to Him if He breaks His word.

The author of Hebrews makes it clear that, since men swear by something greater than themselves and nothing is greater than God, God, in making promise to Abraham, swore by Himself because there is no greater:

For when God made promise to Abraham, because he could swear by no greater, he sware by himself, - Hebrews 6:13

Paul makes it clear that Abraham's children are not those of natural descent but those of like faith with Abraham (all of Romans 4) ... that faith whereby he was credited with righteousness just prior to the covenant in question:

Even as Abraham believed God, and it was accounted to him for righteousness. Know ye therefore that they which are of faith, the same are the children of Abraham. - Galatians 3:6-7

So if we, who have been credited with righteousness by faith (like Abraham), are counted as his children then we have also become objects of the covenant that God made with Himself wherein the only way for us to miss out on the promise is for God to break His oath and suffer the fate of the sacrificed animals:

Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath: That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us: - Hebrews 6:17-18

We are secure in faith because God is a faithful, covenant keeping God.

There many verses that use the word salvation, but they make more sense of you see salvation (check out the Greek tenses and cases of this word) as a processes that will be completed over a period of time.

I can think of the following if I may take the liberty of quoting them out of context!

Old Testament

Psalm 138:8 (NKJV)
²⁸  The Lord will perfect that which concerns me;
Your mercy, O Lord, endures forever;
Do not forsake the works of Your hands.

New Testament

John 10:28-29 (NKJV)
²⁸  And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand. ²⁹  My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father’s hand.

1 Peter 1:9 (NKJV)
⁹  receiving the end of your faith—the salvation of your souls.

Oh, how I dislike the expression "once saved always saved"! It gives a wrong impression that if you just make some sort of “sinners prayer” and ask Jesus to accept you (like you’re doing Jesus a big favour), then you are saved. Nothing else required. Just pat yourself on the back and carry on. Any such notion of “easy believism” is utterly false and misguided.

Let us be clear from the outset – salvation is a sovereign act of God whereby an unregenerate sinner is washed, renewed, and born again by the Holy Spirit:

At one time we too were foolish, disobedient, deceived and enslaved by all kinds of passions and pleasures. We lived in malice and envy, being hated and hating one another. But when the kindness and love of God our Saviour appeared, he saved us, not because of righteous things we had done, but because of his mercy. He saved us through the washing of rebirth and renewal by the Holy Spirit, whom he poured out on us generously through Jesus Christ our Saviour, so that, having been justified by his grace, we might become heirs having the hope of eternal life. (Titus 3:3-7).

Here are some Bible verses in support of the Christian assurance of salvation:

I will not leave you as orphans; I will come to you. Before long, the world will not see me any more, but you will see me. Because I live, you also will live. On that day you will realise that I am in my Father, and you are in me, and I am in you. Whoever has my commands and obeys them, he is the one who loves me. He who loves me will be loved by my Father, and I too will love him and show myself to him. (John 14:18-21)

You, however, are controlled not by the sinful nature but by the Spirit, if the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, he does not belong to Christ. But if Christ is in you, your body is dead because of sin, yet your spirit is alive because of righteousness. And if the Spirit of him who raised Jesus from the dead is living in you, he who raised Christ from the dead will also give life to your mortal bodies through his Spirit, who lives in you. (Romans 8:9-11)

And those He predestined, He also called; those He called, He also justified; those He justified, He also glorified. (Romans 8:30)

Who will bring any charge against those whom God has chosen? It is God who justifies. Who is he that condemns? Christ Jesus, who died—more than that, who was raised to life—is at the right hand of God and is also interceding for us. (Romans 8:33-34)

For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord. (Romans 8:38–39

There are plenty other Bible verses that confirm the assurance born-again Christians have of their eternal salvation. But no person who turns round and denies Christ Jesus as their Lord and Saviour can say they were ever born-again by the new, spiritual rebirth or that they were indwelt by the Holy Spirit.

I hope the Scriptures I've quoted show that there's more to being a Christian than simply making some sort of "sinners' prayer" and saying you will accept Christ Jesus. Commitment is called for. Obedience is called for. A transformed life is called for. You have to BELONG to Christ Jesus and follow Him, regardless of the price.

Once the believer has experienced this rebirth the Spirit that indwells them gives assurance of their salvation. They can cry out, "Abba, Father", confident they have been adopted into God's family. It's a done deal.

Edit: In answer to your comment, “can the done deal get undone along the way (theoretically)?” here is how it works. Genuine repentance results in genuine forgiveness. Once you get up off your knees and dry your eyes you are overwhelmed with the reality of God’s love.

Once God chooses the repentant believer (N.B. God does the choosing) he is justified, not by anything he does, but because God purposed it. Is God about to change His mind and, further down the road say, “Woops, made a mistake with that one. Better cross his name out of the Book of Life”?

The believer who has been reborn has been regenerated. Is he to be un-regenerated because God got it wrong and didn’t know what was round the corner? Where in the Bible does it say the new birth can be taken away? Nowhere – so don’t bother looking.

John 3:15 says that whoever believes in Jesus Christ will "have eternal life." Not, “might have eternal life” or “if you slip up that’s it – you’re history”. If you believe in Christ today and have eternal life, but lose it tomorrow, then it was never "eternal" at all. Therefore, if the born-again believer loses their salvation, the promises of eternal life in the Bible would be a pack of lies.

Jesus made this promise to all believers who turn to Him in faith:

And this is the will of him who sent me, that I shall lose none of all that he has given me, but raise them up at the last day. For my Father’s will is that everyone who looks to the Son and believes in him shall have eternal life, and I will raise him up at the last day. (John6:39-40).

No if’s, not but’s, no hidden default clauses, no traditions of men to follow, no man-made rules to adhere to. God keeps it simple. Humans complicate things in order to control and manipulate.

I’m not about to call God a liar. He is faithful and true and we can depend upon His promises. Bottom line is that God is Sovereign, and He draws us to Him. End of.

Concluding Summary:

I have been saved – in the past – from the penalty of sin – by a crucified Saviour: “For in this hope we were saved" (Romans 8:24). “For it is by grace you have been saved, through faith – and this not from yourselves, it is the gift of God” (Romans 2:8).

I am being saved – in the present – from the power of sin – by a living Saviour: "For the message of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God" (1 Corinthians 1:18).

I shall be saved – in the future – from the presence of sin – by a coming Saviour: "Since we have now been justified by his blood, how much more shall we be saved from God's wrath through him!" (Romans 5:9)