What is the nature of eternal security?

Romans 8:35:

Who shall separate us from the love of Christ? Shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword (KJV)

This passage suggests that nothing will separate us from the Love of Christ, this is something that is assured, this is something that even sin cannot prevent us from receiving, Christ's Love and forgiveness. Nothing can separate us from it, not even our Sin, He loves us regardless of our sin. If we read the passage closely however, we see a few things that stand out. One, the attributes listed in this passage are not sinful actions but things outside of our control, things that may and certainly do happen to Christians in the course of their lives those are persecutions and hardships fallowing in Christ’s footsteps Also, the passage before...

Romans 8:34:

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

Paul is speaking to believers, and suggests that Christs death on the cross did not complete our salvation, rather Christ, who continually intercedes for us as a slaughtered lamb, does so even after conversion, his love for us even in our sin will not falter, he will not divorce us and put us away from his Grace, he wants to know us, he wants us to repent continually and offer himself before the father for that salvation we are promised by our obedience of faith.

So now when we do read 1 Corinthians 6:9-10 we see that it is not Christ whose love is separated from us, but our love that is separated us from Christ and the father by Sin.

1 Cor 6:9-11:

Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortionist, shall inherit the kingdom of God. And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God

Now we have a list that does separate us from the Kingdom of God (Salvation) this list is not one of persecutions and hardships listed as things that do not separate us from the love of Christ, but things that WE DO that separates us from the Christ sacrifice. It is these actions that we choose with our free will that separate us from the free gift, the Grace.

The Nature of eternal security is that we are Completely saved, Sanctified, Justified 100% by Christ’s sacrifice on the Cross. There is nothing that we could have done to merit such a Grace; it is all God through his Son. Having been so sanctified and Justified, An action completed by uniting ourselves to Christ by Repentance through Baptism (of some sort) and Faith (if we are old enough to manifest it). We are now “IN CHRIST JESUS” responsible to stay in that Grace, to abide in Christ, to be his Covenant People, which means to be obedient to his ordinances and walk in his statutes. At the one and only Judgment, Christ, sitting on the right hand of God, will look down at us and say as in Mathew 7:21-23:

Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? And in thy name have cast out devils? And in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity

The words that the apostle Paul uses when speaking of eternal security are "Hope" and "May" with provisions usually concerning our "Abiding" in Christ, not being separating ourselves from the Root. If God's Chosen people can be separated from the Root, be Assured, so once attached to the loving mercy of our lord Jesus Christ, our actions listed plainly in the commandments can and will, without the sacrament of Reconciliation or other Graces known only to God the Father, will indeed separate us from the "Joy of His Salvation". (Psalm 51:12)

It is true that salvation is eternal in its very nature and this implies that abiding in Christ is evidence of salvation:

And being made perfect, he became the source of eternal salvation to all who obey him,

Hebrews 5:9 (ESV)

Abide in me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me.

John 15:2 (ESV)

You did not choose me, but I chose you and appointed you that you should go and bear fruit and that your fruit should abide, so that whatever you ask the Father in my name, he may give it to you.

John 15:16 (ESV)

This abiding in Christ is the act of faith alone from start to finish:

Whoever feeds on my flesh and drinks my blood abides in me, and I in him.

John 6:56 (ESV)

Jesus said to them, “I am the bread of life; whoever comes to me shall not hunger, and whoever believes in me shall never thirst.

John 6:35 (ESV)

This Good News tells us how God makes us right in his sight. This is accomplished from start to finish by faith. As the Scriptures say, "It is through faith that a righteous person has life."

Romans 1:17 (NLT)

What you are considering in this question is the nature of salvation. Eternal security is an outgrowth of the larger Doctrines of Grace.

The subject is huge. In short, though, Eternal Security, as believed by people who hold to the Doctrines of Grace (Canons of Dort), believe that it is a property of salvation. To use your language above, it is by the nature of salvation.

Briefly, why? Read John 6, John 10, and Romans 9. When you do you will see God monergistically working. Faith is a gift that changes the nature of the person so that they repent of their sins. I personally love Romans 8:7-8, "the mind set on the flesh is hostile toward God... it cannot please Him." [paraphrased].

Books that represent the Doctrine of Grace accurately are The God Who Justifies or The Potter's Freedom by James White.

The trouble with the Doctrines of Grace is that there is subtlety to representing them accurately, particularly in how the nature men affects their actions. The books are highly worth it and rather recent.

As it was taught to me as a new Christian, there are two facets of salvation to consider: its objective reality and its subjective experience. John's First Epistle speaks to both points.

John speaks about:

1) Continued relationship to and fellowship with the church (contrasted with those who left)

2) Continually admitting sinfulness, acknowledging Christ's forgiveness, and seeking it (contrasted with those who deny wrongdoing)

3) Avoiding sins that lead to death (a hotly disputed topic)

4) Openly professing that Jesus came in the flesh, from the father, is the messiah, and promised eternal life (as opposed to "antichrists" who deny this)

5) Wrestling with our conscience when it tries to persuade us that we are not saved with the help of the Holy Spirit (from chapter 3:19-24):

"This is how we know that we belong to the truth and how we set our hearts at rest in his presence: If our hearts condemn us, we know that God is greater than our hearts, and he knows everything. Dear friends, if our hearts do not condemn us, we have confidence before God and receive from him anything we ask, because we keep his commands and do what pleases him. And this is his command: to believe in the name of his Son, Jesus Christ, and to love one another as he commanded us. The one who keeps God’s commands lives in him, and he in them. And this is how we know that he lives in us: We know it by the Spirit he gave us."

6) Understanding the implication of possessing the Holy Spirit. 1 John 4:13-16 says:

This is how we know that we live in him and he in us: He has given us of his Spirit. And we have seen and testify that the Father has sent his Son to be the Savior of the world. If anyone acknowledges that Jesus is the Son of God, God lives in them and they in God. And so we know and rely on the love God has for us.

7) Accepting that our security is based on faith (1 John 5):

This is how we know that we love the children of God: by loving God and carrying out his commands. In fact, this is love for God: to keep his commands. And his commands are not burdensome, for everyone born of God overcomes the world. This is the victory that has overcome the world, even our faith. Who is it that overcomes the world? Only the one who believes that Jesus is the Son of God.

8) And as the other commenters said, the importance of keeping away from the worship of idols (1 John 5:21):

Dear children, keep yourselves from idols.

I may not have directly answered what the nature of eternal security or the assurance of salvation is, but those can be learned from the study of how one is to acquire and maintain this assurance. The person who understands the sinfulness of their heart, when they find themselves doing more and more the good and impossible things commanded by Scripture, has experiential assurance and a clean conscience. The person whose conscience troubles them but yet has faith in the promises of God and maintains the confession of their mouth, will believe in their salvation, and that faith will soon restore them by overcoming the evil that temporarily has its hold on them. They possess objective assurance, and that assurance is backed up by the Holy Spirit's ministry, Christ's sacrifice, and the Father's love.