That of course, is the time when salvation is ultimately realized, but the scriptures often and repeatedly express salvation as a done deal.
Eph 2:8 For by grace you have been saved through faith. And this is not your own doing; it is the gift of God,
I understand. However, Paul often uses the present perfect of the verb "have been saved" in order to acknowledge the fact that God never fails to keep a promise. Nevertheless, Paul often speaks about salvation as a future event. Salvation is something we hope for. Salvation is something we inherit. Salvation arrives at the Last Trump.
In order to avoid confusion during discussions about OSAS (Once Saved Always Saved), I often use the present participle "being saved" to acknowledge that our salvation is a process of growth and sanctification, which will be completed in the future. The Bible emphasizes the importance of maturing and becoming more Christ-like, which is a continuous process under God's guidance.
In order to know whether someone has lost his salvation, one must wait until the final bell (so to speak.)
It is not a God-given will to live that prevents lose of salvation; rather, it is in continuing to believe in God that keeps on saved. It is in coming to an unbelief that salvation may be lost.
I think the New Testament teaches another point of view. Continuing to believe is not what keeps us saved. God's salvation is what keeps us believing. Coming to unbelief doesn't indicate a loss of salvation. Unbelief indicates that God is not saving that person. Any other perspective would require that our faith is meritorious and that we are NOT saved by grace.
In Romans 5, Paul argues that our confident expectation of salvation will not go unfulfilled. This is because God has poured out his Spirit within our hearts. So we can ask another question. Will a believer, who is being saved by God and has their heart filled with the Spirit of God, ever be disappointed that God didn't fulfill his promise? The answer is no. Not all believers have the Spirit of God, but Paul guarantees that if God has poured out his Spirit on someone, that individual will never fail to be saved by God.
Even in the event that one loses his salvation through unbelief, it is not God that has failed.
That event isn't possible because, as I say, salvation takes place in the future. No one can lose something he doesn't have.
I am not sure what distinction you are trying to draw between agency and instrument. Those seem to be essentially one and the same. Even so, I do not regard our faith as either the agency nor the instrument through which God saves. Rather our faith is a condition imposed by God upon the one He saves. Faith of the individual is the determining factor on who God elects and saves.
The question is whether faith is a condition of salvation or a symptom of salvation.
I don't believe Peter ever makes the statement that we will never fall away from the faith.
Not explicitly no. But he strongly implies it.
1 Peter 1:3-8
Blessed 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, to obtain an inheritance which is imperishable and undefiled and will not fade away, reserved in heaven for you, who are protected by the power of God through faith for a salvation ready to be revealed in the last time. In this you greatly rejoice, even though now for a little while, if necessary, you have been distressed by various trials, so that the proof of your faith, being more precious than gold which is perishable, even though tested by fire, may be found to result in praise and glory and honor at the revelation of Jesus Christ; and though you have not seen Him, you love Him, and though you do not see Him now, but believe in Him, you greatly rejoice with joy inexpressible and full of glory, obtaining as the outcome of your faith the salvation of your souls.
a living hope . . .
Peter envisions a community of people who have a strong and unwavering hope of being saved. He refers to this hope as "a living hope", which is made possible by the conditions that God has prepared in advance: being reborn and the resurrection of Christ.
to obtain an inheritance . . . reserved in heaven for you.
Peter eliminates all contingency. For individuals who have been reborn spiritually, salvation is not merely a possibility in the future that cannot be predicted with surety. Instead, for those who have undergone this spiritual transformation, salvation in the future is both inevitable and indestructible.
the proof of your faith . . .
We all desire to be certain that we will receive salvation as our inheritance. Salvation is only possible until faith is tested and proven. Once faith is proven, salvation becomes a reality. Peter emphasizes the significance of testing our faith, stating that it's more valuable than gold. For those who have been born again, have the Spirit poured into their heart, and have a proven faith, salvation is guaranteed.