It wasn't in my heart to get into polemics, so I'm sorry for sounding like that. In any event, by adding an application proviso which is not evident in the text, I think it really changes the meaning of the text that really IS there. My understanding of Catholicism is that when an infant is baptized he is saved from original sin. Now, if we stopped right there, then the thing that really concerns me wouldn't apply so much.
But of course, then we have further application as people commit mortal sins through life and take the Eucharist. To me that just doesn't logically match "once and for all". For example, my wife asks me to mow the lawn. I go out and do two rows and then I come back in, declaring "I've finished this week's mowing once and for all". She checks and comes back to me very unhappy. Do I say to her that I simply haven't fully applied the mowing yet? :) Anyway, that's why it's difficult for me to get this, along with the fact that I don't see the second step of application in scripture.
For the mowing example, my not very good first iteration of an answer is you go and mow the grass, and then (for some reason -- known only to women) she fertilizes and waters it. So the mowing activity has to be re-applied.
The reason it's not such a good answer is the problem with the mowing activity which is nothing but temporal (if now downright diabolical) while the forgiving act of Christ is one of those temporal events which leaks into eternity.
Again, the glib inadequate explanation is that God never changes because he's outside of time. On the flipahdeedoodah side, at a particular point in time (was crucified under Pontius Pilate) Jesus wrought the salvation of the world. Right here is where the brain explodes. It is in time and outside of time "all at once".
Mortal sin, deliberate choice of an evil act done with full responsibility and the full knowledge that it was seriously evil,can be viewed (at least) two ways: One way is that it is flipping God off. In our thinking, if you say to God, "Not Thy will, but mine," God respects that, and you get to forsake the full benefits of His fellowship. Another way is that it is like an alcoholic taking not just a sip but a half bottle of whiskey. It sets you out of the "habit" of faith so badly that major mojo is required to get you back.
Now, Where does the major mojo comes from? Recall that I quoted the Catechism "only God can forgive sins." And when/where did He do that? From Our POV, two answers: (a) Before the foundation of the world; (b) "sub Pontio Pilato". Just as on earth and in time and space eternal the power of God is manifested most clearly as a man who cannot scratch his nose or brush away a fly because His hands are nailed back, so the eternal and piercingly joyful love and forgiveness of God is that same man leaking out his life and struggling to breathe.
It is that which washes away original sin and it is that which restores the one who has done what John calls a deadly sin. The same act. The ONCE and for all act which can never and need never be repeated.
Let the eternal God be an ice cold watermelon saturated with Myers Finest Dark Jamaican Rum. Then the sacraments are straws stuck into it to draw out the wonderful nectar.
(Darn that sounds good! I've never done that, but I'd sure like to try someday.)(I mean the watermelon. I've been to confession.)
Now what I wish I could convey persuasively is how this FEELS like a redemptive sacrament. It is not that God is limited to forgiving mortal sins after Baptism this way. God is like the pay master who looks through the book to find a way to give us a little more, as opposed to what we think of Him too often (even if we're not really aware that we think it) which is that he's the pay master who is intent on finding out how to hold back as much as possible.
But when we go to confession, at least when I go to confession, SURE there is a reluctance, feeling of guilt and shame and whatnot. But at this point in my life it's more about, say Physical Therapy (one of my fave metaphors: It hurts so good!) I WANT not only the act of restoration and the prescribed penance, but I also want to LIVE what I believe, at least to the extent that I believe that my sinfulness is not the most important thing about me, because God's love is the most important thing about me. (I mean, a better way to live it would be not to sin, but evidently I blew that one.) When I go to the priest, I am drawing on the confidence that one day God will share with me a fuller knowledge of the great horror of my particular sins and the greater Joy of His love -- and I will be able to see my sins as occasions of grace, as "happy faults", happy because they received so great a champion and rescuer.
So confession draws on the once for all saving act of Christ AND is a way I step (again) into the grace and freedom of that act.