Eternal security is the doctrine of “once saved, always saved.” It is the belief that once a person is saved, nothing he nor anyone else does can take away his salvation. This is in contrast to conditional preservation, which states that a person can lose his salvation by some subsequent sin.
Proponents of the doctrine of eternal security see this reaffirmed throughout Scripture:
John 6:35, 37–39 And Jesus said unto them, I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst. . . . All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out. For I came down from heaven, not to do mine own will, but the will of him that sent me. And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day.
Jesus promised that those who come to Him will never be spiritually hungry or thirsty again and never be cast out. He said that of all the people that the Father gives Him, He will not lose any of them. Nothing is conditional.
John 5:24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
Again this says those who believe on God have eternal life and will not be condemned. It doesn't say we will not be condemned if we continue to believe or if we continue to do good works. It says we shall not come into condemnation and are already passed from death to life and already have our eternal life.
John 10:27-29 My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand.
Again, Jesus says that those who follow Him have eternal life and will never perish and that no one can take them out of God's hand.
A common retort to this verse is that while no one can take us out of God's hand, perhaps we can jump out on our own, either through apostasy or just general sinfulness. Firstly, this new metaphor contradicts the Bible's metaphor of the Devil being a carnivorous creature trying to steal us from Christ the Shepherd's fold. John 10:12; 1 Peter 5:8 In a biblical sense, someone losing his salvation would be akin to the Devil stealing one away. Yet Romans 8:38–39 assures us that “neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.”
Secondly, this misconstrues who it is that our salvation depends on. We are saved by grace through faith as a gift of God, not through ourselves. Ephesians 2:8 The gifts of God are irrevocable. Romans 11:29 And it's not that we were saved by grace, but we are continuing in salvation through God's gift of grace.
If we are not saved by works, why would us continuing to be saved depend on our works? Paul seems to directly bring this up in Galatians 3:3, “Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh?” He is saying that if we were saved by faith, why we think we continue to be saved and grow in Christ through our own efforts? Just as it's God who saves us, it's God who keeps us saved and growing more like Him. 1 Peter 1:5; Jude 24
Thirdly, salvation is more than just God giving us a pass into heaven. It is an adoption into the family of God. We, who are naturally children of Satan, become the sons and daughters of God. Ephesians 2:2–5; Romans 8:15–17 This is a permanent change in our identity. No matter how much a son may disappoint his father or even rebel against him, he can never make himself not a child. He can certainly break the fellowship with his father, but he can never break the relationship. Likewise, we can never break our relationship with God. We can never make ourselves not His children. No matter how much we rebel, God will always be our Father and will always be waiting for us to return to Him. Luke 15:11–32
It is even more certain that God will never cast us out Himself. Once we repent and come to God, we can rest sure that he will never reject us thereafter.
John 6:37 All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.
The New Creature
If we come to Christ and we are made His children and He saves us by His power, what could make us unsaved thereafter? Is our salvation dependent on our works? Will God cast us out after all if we sin? Such an idea would break many of God's promises. He says he gives whoever believes on Him should not perish but have eternal life. John 3:16 Again, John 5:24 says that those who believe on God have eternal life and will not be condemned. At the moment of salvation, we are justified and declared righteous. Romans 5:1 Jesus' righteousness is imputed to us when we believe. Romans 4:11, 23–24
Futhermore, we are then new creatures; old things are passed away. 2 Corinthians 5:17 This hope and reassurance makes little sense if we can just go back to being the old creature. They wouldn't really be passed away.
Sealed with the Spirit
This brings us to the strongest argument for eternal security: the Holy Spirit. Earlier in 2 Corinthians 5, it says:
2 Corinthians 5:5 Now he that hath wrought us for the selfsame thing is God, who also hath given unto us the earnest of the Spirit.
The Holy Spirit is the earnest of our salvation. An earnest is a down payment. It is a guarantee that the rest will be paid. The Holy Spirit is God's guarantee that He will complete the work He has started in us. Philippians 1:6
We are bought with the price of Christ's death, and the spirit is indwelling us as temples of God is the sealed proof that the transaction is complete. 1 Corinthians 6:19–20; Ephesians 1:13–14 We are sealed by the Holy Spirit unto the day of redemption. Ephesians 4:30 Though we may grieve the Holy Spirit, He will never leave us.
What of the problem of Christians sinning then?
In Psalm 51:3, 10–12 David mourns his sin and laments his loss of joy. He pleads for a clean heart and a right spirit and fellowship with God and for the joy of his salvation to be restored. Yet it's for the joy of his salvation, not his salvation itself. He doesn't ask for his salvation to be restored, because he knows that it is secure. Before Pentecost, the Holy Spirit did not indwell each believer, so he feared losing God's power and direction in his life, but even without the Holy Spirit, he knew that God would not condemn him. If David was secure in his salvation after committing murder and adultery, what sins could be to grievous?
In 1 Corinthians 3, Paul describes the Corinthians as baby Christians who failed to grow and instead were carnal and full of envy, strife, and divisions. Yet Paul still calls them the temple of God. He says even if their works are founded on something other Christ and they will be burned up as worthless, yet the people themselves would be saved.
Romans 8:1a says, “There is, then, now no condemnation to those in Christ Jesus”. This is a non-restrictive clause. Again, there are no conditions. It says we will not be condemned for our sins, not that we will not be condemned unless we do something really bad. In fact, what does it say will happen if we sin? Does it say God disowns us as His children?
Romans 8:26–31, 33–34 Likewise the Spirit helpeth our infirmities: for we know not what we should pray for as we ought: but the Spirit itself maketh intercession for us with groanings which cannot be uttered. And he that searcheth the hearts knoweth what is the mind of the Spirit, because he maketh intercession for the saints according to the will of God. What shall we then say to these things? If God be for us, who can be against us? . . . . Who shall lay any thing to the charge of God's elect? It is God that justifieth. Who is he that condemneth? It is Christ that died, yea rather, that is risen again, who is even at the right hand of God, who also maketh intercession for us.
1 John 2:1–2 My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous: And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world.
Rather than saying God will disown us if we sin while saved, it says Jesus rather advocates on our behalf and the Holy Spirit intercedes for us. We have a high priest who is touched with the feeling of our infirmities. Hebrews 4:15–16 No matter what we do, God is still for us and will uphold us. Isaiah 41:10 Satan is the one who accuses the brethren of our sin, not God. Revelation 12:10 Jesus advocates for us.
That's not to say God will just look the other way with our sin and let us remain as we were before becoming His child. His goal is to make us like Christ. Romans 8:28–29 says that those who love God are already destined to have this goal fulfilled. It's so sure that we will become holy like Christ that Romans 8:30 says in God's eyes we are already justified and glorified. There's no question that we might fail for God has already determined and provided for our success.
Yet the means of this sanctification is discipline. As earthly fathers chasten their children to make them better, so God will discipline his wayward children. Hebrews 12:5–7 Receiving earthly consequences for our sins is not condemnation nor some evidence that we're not saved. Quite the opposite, it's the proof of our salvation. If we do not receive discipline when we sin, it's a sign that we are not God's children. Hebrews 12:8 (You wouldn't discipline someone else's child, would you?) Although discipline is painful, it is for our good. Hebrews 12:11 This is what God means when he says “all things work together for good to them that love God” in Romans 8:28.
If people sin without consequence or regret, they never were Christians. 1 John 2:19 As 1 John 2:4 says, “He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.” It doesn't say he lost his salvation; it says he's lying about ever knowing God. Those who are truly saved can rest secured that no matter their struggles, God will finish the work He started in them.