For me, the issue of eternal security (as well as most of the other seemingly contradictory doctrines) rest upon the basis of does Scripture contradict itself. If we believe that Scripture teaches that it cannot contradict itself because it is true in its entirety (Ps. 19:7-9; 33:4; 119:160, Jn. 17:17, 2Tim. 3:16-17), then Scripture isn't the issue, rather our interpretation of it is. Does Scripture teach eternal security? I would say yes. Does Scripture teach conditional security? I would also have to say yes. But this then poses a contradiction, which means how I and others have interpreted these passages are off.
I personally hold to the position of eternal security. The reason why I hold to eternal security rest primarily in several interwoven passages: John 6:39; 10:14-16, 27-29; 14:16-17, Romans 8:29-30, 1Corinthians 1:8, 2Corinthians 1:21-22, Ephesians 1:5-6, 13-14, 1John 3:7-9; 4:13. Time does not permit for me to explore in this setting contextually all the passages I have cited. Nevertheless, from these passages we find that...
(1)Jesus will not lose (in context "let perish")(*1) anyone that comes to Him,
(2)Jesus will bring all His sheep into His one fold,
(3)absolutely nothing and no one can take away Jesus' sheep from Jesus,
(4)believers receive the Holy Spirit, and He is in them forever,
(5)believers are predestined by God to be conformed to the likeness of Jesus,
(6)believers are foreknown, called, justified, and glorified all by God,
(7)God will confirm (in context, "establish")(*2) believers blameless (literally "without fault in His sight")(*3) to the end,
(8)God has given and sealed (literally "stamped for ownership")(*4) believers with the Holy Spirit as a pledge (in context, "a deposit given as a guarantee")(*5),
(9)God has predestined believers to adoption as His children to the praise of the glory of His grace,
(10)believers have been sealed(*4) in Jesus with the Holy Spirit as a pledge(*5) of our inheritance to the praise of God's glory,
(11)a person cannot be truly born-again and still live in (practicing) sin because God's seed is in them, and
(12)believers have the assurance of knowing they're in God because God has given them the Holy Spirit.
I cannot read these handful of passages and believe somehow I, even with my free-will, can be truly born-again and still lose my salvation. I'm not denying free-will. I know full well I have a responsibility in working out my salvation and discipleship, in walking in the Spirit, in standing firm in the faith, and so forth. But just seeing from these handful of passages how active God is in keeping what He Himself has redeemed, it would be arrogant of me to say I can be truly born-again (regenerated) at one point and then, despite God's seed, seal, pledge, predestination, foreknowledge, call, justification, authority and power not to lose what He has, obligation to gather all of His sheep, confirming believers to the end, and the Holy Spirit being in us forever, I can become unborn-again (unregenerate). I acknowledge that their are passages that do speak to this very thing. However, while I may not fully understand what they mean just yet, I stand on the basis that Scripture cannot contradict itself, thus those passages somehow correlate with the truth mentioned in the above passages rather than the other way around. Those passages above do not fit whatsoever with conditional security. For example, to be able to somehow reverse or resist predestination contradicts predestination. I believe this is where our free-will and God's sovereignty work hand-in-hand. We do our part of working out and walking out our discipleship. God does His part of keeping us and sanctifying us along the way.
True born-again believers will bear fruit of the Holy Spirit in their lives and will look more like Jesus over time (Jn. 14:16-26; 15:1-17, 26-27, Rom. 6:12-22; 8:9-14, Gal. 5:16-26, Eph. 2:10, Phil. 2:12-16, 1Thess. 4:1-8, Tit. 2:11-14, 1Pet. 1:13-19, 2Pet. 3:18, 1Jn. 2:3-6); otherwise, they cannot say they have been truly born of the Holy Spirit if there is no evidence of the Holy Spirit in one's life. There is no one basic standard every Christian must reach. Each Christian who truly has the Holy Spirit will bear His fruit, but we all grow and mature differently—that is, at different times and in different areas. But there will be growth, for growth and maturity is a fruit of sanctification. Can true born-again believers fall back into sin? Sure. We see clear examples in Scripture. However, repentance is ever-present for a true believer because the Holy Spirit is ever-active in them. True believers may fall, but it will only be temporarily, not completely (Ps. 37:23-24, Prov. 24:16, Rom. 6:17-18, Phil. 1:6, Jude 1).
The doctrine of assurance is closely connected to the doctrine of eternal security. Our assurance of our salvation is the Holy Spirit. Likewise, the Holy Spirit is also one of our assurances of our eternal security, for He is our seal and pledge from God. Furthermore, in my estimation, the doctrines of eternal security and assurance do not create complacent, lethargic Christians, rather sin and selfishness does. Yet, on the contrary, the work of the Holy Spirit in believers' lives is to produce His fruit, Christ-likeness, and growth/maturity.
To be honest, I feel bad for the believers who are not so solid in their faith, because these types of arguments can very well have them second guessing what they believe. We have to do a better job of disagreeing in love and working toward some type of common ground amid controversial doctrines.