I'm reading chapter 21 of The Infinite Atonement by Tad R. Callister and it's INCREDIBLE.
So, my understanding of what Callister explains is that the atonement isn't just a redeeming power, restoring what was lost in the Fall of Adam & Eve, it is also an affirmative power, a positive process of exalting, that absolutely requires mercy. Paying justice is not enough to be perfected, we need the affirmative process of "becoming a saint" "through the atonement of Christ the Lord" (Mosiah 3:19). We need mercy! Justice "is neutral, always neutral" and we only pay the balance of a debt to justice, so there is no positive outcome. That's why repentance and mercy are ESSENTIAL. Repentance involves "the internal desire of man ([which seems to come as a result of mercy being extended]) combined with the external power of God, so merging in miraculous harmony that it enlarges, endows, and enlightens the human spirit" Yes, it satisfies justice, but by a POSITIVE process that actually adds to our souls. We don't simply endure the payment of a debt but instead open a humbled heart to the Atonement, to the mercy of our Savior, which changes our nature for the better and "gives us the capacity to live the celestial law."
I guess I knew this, but I'd never really put all that together that way, that "if we decline the Savior's invitation to let him carry our sins, and then satisfy justice by ourselves, we will not yet have experienced the complete rehabilitation that can occur through a combination of divine assistance and genuine repentance."
p.s. READ THE BOOK!
p.p.s. C.S. Lewis said that the atonement "would have occurred for Glorification and Perfection even if it had not been required for Redemption."