When the righteousness of Christ is first revealed to a sinner as the only way to be accepted by God, he is amazed and full of wonder and rejoices greatly. So he heartily approves of this righteousness because it reveals the glory of the wisdom of God (1 Cor 1:21). He sees what darkness he was in. He looked into himself and found only sin, horror, fear and tremblings. He looked up and saw nothing but wrath, curses and vengeance. He saw that God was holy and righteous and that no unclean thing could abide with him. He saw that he was a poor, vile, unclean and sinful creature and he could not see how a holy God and a sinful creature could be reconciled. But in the righteousness of Christ, a world of wisdom is opened, dispelling all difficulties and darkness and revealing how reconciliation could actually happen (Rom 11.33; Col 2.3).John Owen, Communion with God (Banner of Truth), pp.141-142
What grace is revealed in this righteousness of Christ! The sinner does not have to earn it. God everywhere assures us that this righteousness is of grace (Rom 11.6; Eph 2.7-9). It is given to us by grace. It is from the riches of grace and kindness that this righteousness has been provided. So believers rejoice in this righteousness because it is ours by grace only.