Thank you for the question you submitted via the Internet regarding eternal security. Simply stated, the Bible teaches that we are saved by the grace of God, not on the basis of our personal merit. When we realize that it is impossible for us to ever gain the favor of God through personal righteousness, we are able to also grasp that our relationship with God is not preserved that way either.
I emphatically and dogmatically believe that a person who is genuinely born again cannot ever know separation from God. I believe in the doctrines of eternal security and the perseverance of the faith. However, not everyone who claims to know Jesus Christ actually does. There are many who profess to believe, but who have no relationship with the Lord. Their lives are not changed, they have not abandoned their sin, and are characterized as those who "practice lawlessness."
Once a person has been quickened by the grace of God, they can never be "lost" and will be delivered eternally. Thus, salvation is clearly a work of God provided us through grace as a gift. It is not a work of man since he is spiritually dead.
However, it is true that there are some people who profess to know Christ, but continue in their devotion to sin. 1 John 2:4 speaks to this issue clearly: "The one who says, `I have come to know Him,' and does not keep His commandments, is a liar, and the truth is not in him." Hence, we see that God draws a distinction between those who "profess" Christ as opposed to those who actually "co nfess" Christ. For instance, 2 Peter 2:20 describes those "False Teachers" who were professing themselves to be followers of Christ having escaped the defilements of the world, but then rejected what they professed, becoming slaves of corruption ( v. 19) and showing their true, natural unchanged condition ( v. 22). The Apostle John describes this same situation when people who appeared to be Christians abandoned the faith in 1 John 2:19: "They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us." It is from this passage and others that we understand the doctrine of the "Perseverance of the Saints."
The issue rests on the competence of our God to keep His Word when He states that "All that the Father gives Me shall come to Me, and the one who comes to Me I will certainly not cast out." (John 6:33). The most important factor is not whether one professes to be a believer, but whether they ACTUALLY ARE a believer. There are those who profess, and yet will be denied by Christ on judgment - cp. Matthew 7:20 ff. The important factor is whether one has a personal relationship with Christ Jesus that makes a daily difference in the way a person lives.
I trust this helps.