how could it have been eternal life to begin with even if we lost our salvation for one second?
Why not? We already exist eternally anyway, finally either in heaven or hell, hell being a sort of “eternal death”. When we’re born again/saved we’re justified,
made just, forgiven, cleansed, washed, made new creations, filled with righteousness, not a merely
imputed one as per Protestantism generally speaking. This righteousness or justice consists basically in the virtues of faith, hope, and, most importantly, love, even if these reside in us only in nascent or seedling form at first, waiting to be expressed, tested, refined, confirmed, strengthened and grown. IOW, we’re given the life of God, a communion entered into via faith, and now we’re expected to walk and live like it, to remain in Him and He in us, to struggle and fight the battle against sin, “invest” our talents (the Parable of the Talents sheds great light on this whole dynamic), to strive, to be vigilant, to persevere, to do “for the least of these”, feeding the hungry and clothing the naked, to love, to be perfect, to be holy, to do good.
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Make every effort to live in peace with everyone and to be holy; without holiness no one will see the Lord.” Hen 12:14
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To those who by persistence in doing good seek glory, honor and immortality, he will give eternal life.” Rom 2:7
Love is the primary aspect of man’s justice or righteous, it’s the goal, love of God and neighbor, which is why the Greatest Commandments are what they are and why the Church can teach that, at the end, we’ll be
judged on our love. Love fulfills the Law (Rom 13:10) and produces obedience by its nature. Mortal sin directly opposes and destroys love by
its nature, it’s to turn directly back away from God.
Of course we can lose our state of justice and therefore our salvation, by acting
unjustly. Scripture says that our own sins won’t even be forgiven unless we forgive others
their sins. John 15 says we can be branches grafted in but later cut back off, Heb 6:4 tells us it’s possible to taste of the heavenly gift but later reject it, returning to sin, life in the flesh.
Salvation is not simply a one time event; that would be absurd. It’s “worked out“ as per Phil 2. Man’s will is never totally uninvolved. We’re judged based on how we did, how we loved, properly understood, with the gifts given: time, opportunity, experience, revelation/knowledge, and grace, with more demanded of those given more (Luke 12:48).