Let's start here. You believing (in Christ) did not save you. God's determination to save believers is what saved you (Jn.3:16). It is a gift. We were dead in our sin, we had no spiritual life and as such, were unable to relate to God who is Spirit. We were born utterly apart and oblivious to Him. It was God who determined to make us spiritually alive (born from above). We had no say in that. You don't earn gifts but you constantly raise verses that deal with our walk to try and show we can lose our salvation from death like Paul talking about "finishing the race" It is
this salvation from death that Christ died for. The wages of sin is death. He died so we could be alive. We had no legal, moral or spiritual standing to ask God for life. There is little point speaking of holiness (sanctification) without first being made alive. Salvation from death is an instant event and it is this salvation that the doctrine of eternal security is referring to as something that can never be lost. Once spiritually alive, always spiritually alive (Jn.11:26), God knows who He is saving (Matt.7:23) Salvation from the corruption and decay of death (sanctification) is an ongoing process that will not be complete until the day of Resurrection. This can be done well or poorly. One's walk can either produce gold, silver and precious gems or wood, hay and stubble. The former will reap the rewards as noted in the letters to the Churches in revelation and other places but those who produce the latter end up with no crowns but still get a Resurrection body, they are still saved from death.
1Cor.3:11-15
11 For no other foundation can anyone lay than that which is laid, which is Jesus Christ. 12 Now if anyone builds on this foundation with gold, silver, precious stones, wood, hay, straw, 13 each one’s work will become clear; for the Day will declare it, because it will be revealed by fire; and the fire will test each one’s work, of what sort it is. 14 If anyone’s work which he has built on it endures, he will receive a reward. 15 If anyone’s work is burned, he will suffer loss; but he himself will be saved, yet so as through fire.
There are those who say this only applies to teachers of the word but even if that is so, the passage doubles down on how secure our salvation is because teachers are held to a stricter evaluation and if they get it completely wrong and are still saved, how much more for us whose evaluation is more lenient.
Until such time as you comprehend we are talking about two different and distinct phases of salvation when discussing the doctrine of eternal security, we will only keep going round in circles.
Phase 1 - salvation from death, a result of the Gospel (Cross event - death and resurrection of Jesus Christ) - God's doing and therefore utterly secure and cannot be lost. Occurs instantly at the moment one finds themself believing the Gospel.
Phase 2 - salvation from the corruption and decay of one's sin, the world and the devil. Known as sanctification. A strict way of walking laid down by God as the only way to sanctification. ie. By grace, through faith in the filling of the Spirit. - requires our co-operation and therefore has an element of potential success and/or failure. We can fail to mature and lose the complete contentment that Paul talks about in Phil.4:11. This is something that
must be learned, it is
not a gift given from God's sole determining, we have a role in this. Not all believers will learn how to be truly content in all things and so will not be free of corruption.
Phase 3 - eternal salvation - from the Day of resurrection and onward. Most of which is a mystery but we know we will receive a body like His and we will be forever with the Lord.
All sin means all sin. Every sin that has been committed beginning with Adam's to the last person ever born, is forgiven. That forgiveness is
in Christ. If you want to know (experience) that forgiveness and the salvation it brings, there is only one place to be,
in Christ. Anyone not found in Christ will never know what it is like to be saved but it is due to their negative attitude to Christ (Jn.3:18), not because God is holding their sin against them.
All sin has been dealt with, believer and unbeliever alike.
John 1:29
The next day John saw Jesus coming toward him, and said, “Behold! The Lamb of God
who takes away the sin of the world!