Or to say what I said below in another way...Augustine, Biel and Calvin all agree that ontological renewal is necessary, but all have different ways of getting there.
For Augustine, God observes our need and offers us a participation in Christ, resulting in a reverse incarnation. He became as us so that we could become as him. This is justification for Augustine.
For Biel, God observes our need and establishes an agreement wherein if we do all that is within us to perform a true act of love toward God, he will grant us the grace of ontological renewal. This is justification for Biel (Pelagian, to be sure).
But for Calvin, God's justice stands in the way of him granting the grace of ontological renewal outright. God must first be propitiated in relation to his justice. God sends Christ to pay the just penalty for our sins, removing the legal barrier between God's justice and God's desire to be gracious. This is justification for Calvin.
Calvin's doctrine of justification adds a layer to his soteriology not present in Augustine and Biel. For Augustine and Biel, there is no need for God to be propitiated prior to his offering us the grace of ontological renewal. God's justice does not stand in the way of his mercy. But for Calvin, God's desire to be gracious and his ability to be gracious are at odds. Thus Christ's death is necessary in Calvin's system in ways not seen in Augustine and Biel. Herein lies a major difference in their respective soteriologies, and consequently their respective doctrines of justification.
I agree with Calvin that Christ's death was necessary. And I agree with Augustine that justification is primarily about ontological renewal. And I agree with Biel that God is not bound by human standards of justice. And I think that Athanasius charts a course wherein all three of these can be brought together.