Here are two favorite quotes regarding how we can be assured of the reality of God’s justification.
The first is from Geerhardus Vos in his Grace and Glory: Sermons Preached in the Chapel of Princeton Theological Seminary (Solid Ground, 2007):
“Among all the realities of the invisible world, mediated to us by the disclosures and promises of God, and to which our faith responds, there is none that more strongly calls into action this faculty for grasping the unseen than the divine pronouncement through the Gospel, that, though sinners, we are righteous in the judgment of God. That is not only the invisible, it seems the impossible; it is the paradox of all paradoxes; it requires a unique energy of believing; it is the supreme victory of faith over the apparent reality of things; it credits God with calling the things that are not as though they were; it penetrates more deeply into the deity of God than any other act of faith.” (135)
The second is from Robert Kolb and Charles Arand in the The Genius of Luther’s Theology: A Wittenberg Way of Thinking for the Contemporary Church (Baker Academic, 2008):
“Those who see this form of forensic justification as merely a legal fiction do not share Luther’s understanding of the power of the Word of God. The reformer knew that from the beginning of the world, God determined reality by speaking. Therefore, he was certain that God’s word of forgiveness created a new reality in the life of the sinner. The reformer could not explain the mystery of evil and sin continuing in the lives of those God had claimed as his own in baptism. But he did not doubt that when God said, ‘Forgiven,’ the reality of human sinlessness in God’s sight was genuine and unassailable. God’s children must live with the mystery of the continuing sin and evil in their lives as they engage in the battle against their own sins. But they have no warrant to doubt that God has established the mightier reality of their innocence in his sight. And what he sees is real because he determines reality.” (154-155)