The key question of the Reformation was not, “How can I be a better person?” but “How can I, an evil person, be accepted by a holy and righteous God?” Do I have to get better before I can be well? The Reformation answered, with the New Testament, that we would always be “ill,” spiritually and morally, in this world; that we were, even as Christians, still sinners. Nevertheless, God declared us righteous while we were still unrighteous. Grace, then, is not first a power infused to help us become good, but God’s acceptance of us as good even while we are still evil. This did not eliminate the moral renewal of sinners (new birth and sanctification). In fact, the Reformation argued that in our union with Christ we receive both justification and sanctification. They are inseparable. However, they are not the same.
The medieval church had collapsed the verdict of justification into the process of sanctification: by cooperating with grace, you are actually made just (righteous) and merit a final justification at the last judgment. Because most of us remain imperfect at death, some time in purgatory is necessary before we can be received as fully justified in heaven. The Reformers rediscovered the Good News that justification is not something that we aim for in life but something that we already have completely and perfectly in Christ through faith alone. Therefore, we are to live in gratitude. Justification is the basis, not the goal, of our growth in grace.
Horton, M. (2011). Putting Amazing Back into Grace: Embracing the Heart of the Gospel (pp. 143–144). Grand Rapids, MI: Baker Books.