In studying Roman Catholic theology it took me so so long before it dawned on me that justification is not viewed as an event as it is in Protestant theology. Why is this single concept so critical in dividing Catholic from Protestant theology?
Why Protestants see themselves as being able to break out of the cycle of sin through their personal affirmation that Jesus is their Personal Savior is more or less a mystery to Catholics.
Why Catholics think the mere act of eating what appears to be a piece of Bread can renew their life within them and give them a share in Sanctifying Grace is a more or less a mystery to Protestants.
It's probably decisive because Catholics are pretty convinced they're right and here's why.
As God's chosen people we (All Christians) are not a whole heck of a lot different today than they (The tribes of Israel) were 3000 years ago. King David for instance, went through all the trouble of being a very good, wise and courageous youth, only to give in to temptation and then repent of it and die a good and saintly man.
The cycle is too alliterative to be of God, but this is what it looks like
The act that draws you away from God
Servitude to sin
The time you spend getting further and further away from God
The cry for help that brings you back to God
God's response to your cry
The period of time in which you get to decide whether or not to sin again.
Chances are pretty good you're going to go back up to step 1. But there's no going from Step 5 to Step 4 (because those who are not sick don't need a physician) and there's no point going from Step 5 to Step 3. God's grace will keep you from going from Step 5 to Step 2. But your free will and passions may move you from step 5 to step 1.
As we grow in holiness we can extend our time inside of Step 5 land. Mother Theresa spent most of her life there!
It's a process, because our earthly lives are finite, but it's more like a big ball of yarn all rolled up. We're not Buddhists; our spiritual development isn't circular, because it isn't infinite. But it is more or less round and messy and it's not instantaneous.
Protestants seem to on the other hand, cling to a translation of John 3:7
Do not be amazed that I told you, ‘You must be born from above.’
which says "again" instead of "from above". And regardless of the translation, this signifies some sort of a birth event that takes place after which you're saved once and for all.
Last week I heard of Go Ask Your Father. The good Reverend Know-It-All expound on this. He Pentecostal cited a friend of his who said neither Catholics nor Protestants understand what it means to be "born" in this context. That what we talk about as "born" is really "conceived". Because being born is a painful excruciating event (I've never done it myself, but I've been rather intimately involved in each of my children's births and he's right. It's tiring, it happens unexpectedly, it is an event, but it's not a life changing event because the life has been in existence, hidden for 9 months already.
So, maybe what Jesus was saying was that you're going to have to go through life and have struggles and problems and carry your cross to unite your birth (through baptism) to his death. Which, incidentally, is exactly what St. Paul says happens when we're baptized, regardless of if we're baptized as adults or wee infants.