What is the doctrine of eternal security?

When we speak of the doctrine of eternal security, we’re using a popular description of a classical doctrine that we call the perseverance of the saints. What it means is that once a person has become quickened by the Holy Spirit, born of the Spirit, and justified through faith in Christ and therefore placed in a state of salvation, that person will, in fact, never lose his salvation. That is a very controversial point within the context of historic Christianity.

There are many Christians who do not believe that once a person is in a state of grace, he will abide in that state of grace. The Roman Catholic Church, for example, historically teaches the distinction between venial and mortal sins. Mortal sin is defined as being mortal because it has the capacity to kill or to destroy the justifying grace that is in the soul, and such a sin makes it necessary for a person to be restored to justification through the sacrament of penance. Other Christian bodies also believe that it is possible for a Christian to lose his salvation. Advocates of eternal security say that our salvation is secure once it is wrought through faith and that nothing shall separate us from the love of Christ. It is based on some passages in Scripture, such as Paul’s teaching in Philippians. It is said that, “He who has begun a good work in you will perfect it to the end.” Also, the Scriptures talk about the work of the Holy Spirit in the Christian life. Not only does the Spirit regenerate us, or quicken us, starting the whole process of Christian living, but as the Bible tells us, God gives to each Christian the sealing of the Holy Spirit and the earnest of the Holy Spirit. That term is a little bit obscure in everyday vocabulary, although when we buy a home the real estate agent might ask us to make a little down payment that we call earnest money. That is an economic phrase we use, and it is used in Scripture in that same way. An earnest was a down payment, an absolute guarantee that the balance would, in fact, be paid. When God the Holy Spirit puts a down payment on something, he doesn’t renege on the payments. God the Holy Spirit does not give you an earnest that becomes less than earnest. He’s deadly in earnest to finish what he has begun with you. Also, the concept of being sealed by the Spirit draws from the ancient language of the signet ring of the emperor. When something was sealed and affixed with the imprimatur of the king or the owner, then it became his possession. I think we have to make this qualifier: If it were up to us, I don’t think any of us would persevere, and we would have very little to be secure about. However, the concept as I understand it biblically is that God promises that no one will snatch us out of the hands of Christ, that he will preserve us.

Sproul, R. C.. Now, That's a Good Question! (pp. 136-139). Tyndale House Publishers. Kindle Edition.