The issue of eternal security is one of great disparity among Christians. There are passages that seem to indicate at first glance that salvation can be lost. But as I have searched these further (such as Heb 6 and Gal 5:4), they are not referring to personal salvation. What I do see is an extremely strong case for the security of the believer. Now, the term eternal security can be misleading. It itself is not in the Bible. Thus, many misuse the terminology to think that it implies a "get out of hell free card" which allows them to live as they please and still go to heaven. This is not the teaching at all in the Scriptures. The Scriptures teach that, though a believer can rebel and shipwreck his faith (1 Timothy 1:19), Christ will never forsake them (Hebrews 13:5). In fact, it is my belief that a rebelling believer will spend much time in internal misery. They must since the Holy Spirit is not going to stop reminding them of their sin until they deal with it. Thus, they will have no prayer life, no awareness of God's presence, no real hope, and no fellowship, assuming the church disciplines them as per Matt. 18 and turns them over to Satan for the destruction of their flesh (1 Cor 5:5) (sin leads to death, hurt, and destruction). The carnal, unfaithful Christian is the one which will suffer much loss when he stands before Christ's judgment (2 Cor 5:10, 1 Cor 3:15). 1 Cor 3:15 says, "If any man's work is burned up, he will suffer loss; but he himself will be saved, yet so as through fire." That we ourselves will be saved despite the realization of the seriousness of a wasted life in Christ resulting in a loss of eternal rewards is our hope in Christ.
If keeping our salvation was a matter of our performance, we would all fail. James 3:2 says, "For we all stumble in many ways." Upon what standard can one know if he is saved if it is based on performance? How many good works are enough and how many bad works are too many? What about the person that lives an obedient life only to sin and die before having a chance to ask forgiveness? The issue is that our salvation is locked up in Christ, not our ability to perform good works. The true believer will have good works, some more than others (Matt 7:20, James 2:17). There will be a measurable, noticable difference (the message of 1 John). 1 John 5:13 in fact sums up that John's purpose was to show us that we can know if we are saved based upon our love and lifestyle. Works should be evident, but nowhere are works in the Bible ever sufficient to save a person. They are evidence of Christ at work having saved a person and now sanctifying them. The issue is not us as much as it is Christ.
The following Scriptures must shape our view of eternal security:
Hebrews 12:2 "fixing our eyes on Jesus, the author and perfecter of faith, who for the joy set before Him endured the cross, despising the shame, and has sat down at the right hand of the throne of God."
He started our faith ultimately and will finish it. Note also that He is at the right hand of God. Who else is there? Us! (Col 3:1, Eph 2:6). We are presently positionally and legally seated with Christ in heaven. His inheritance of the saints is sure.
1 Peter 1:3-5, 13
"3Blessed be the God and Father of our Lord Jesus Christ, who according to His great mercy has caused us to be born again to a living hope through the resurrection of Jesus Christ from the dead, 4to obtain an inheritance which is imperishable and undefiled and will not fade away, reserved in heaven for you, 5who are protected by the power of God through faith for a salvation ready to be revealed in the last time. 13Therefore, prepare your minds for action, keep sober in spirit, fix your hope completely on the grace to be brought to you at the revelation of Jesus Christ."
If it was up to me, I would have no hope. I am just not good enough to be sure that I could preserve my salvation, especially given that all of us have hidden sins (Psalm 19:12) that we are not aware of. The adjectives Peter uses to describe our salvation are imperishable, undefiled, unfading, and reserved. We are protected in Christ. This is a strong argument for eternal security.
"My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of the Father's hand."
Again, the issue is God's grace, not human merit and ability.
"and these whom He predestined, He also called; and these whom He called, He also justified; and these whom He justified, He also glorified."
Those who are glorifed with Christ in heaven are those who are saved. It is a done deal from God's perspective Who exists outside of time.
There are many other Scriptures (Eph 1:13, 2 Cor 1:22, etc.) which in particular emphasize the Spirit being a guarantor of our salvation. Again, the emphasis is on God keeping us, not we being able to keep ourselves. This makes sense, given that we were unable to save ourselves, not even desiring the things of God, save for His grace.
"All that the Father gives Me will come to Me, and the one who comes to Me I will certainly not cast out."
Who else will be cast out? Satan. (John 12:31). We, however, are adopted as children of God. We cannot be disowned not because we will be perfectly obedient children but because God loves us and has adopted us.