"By Jesus' sacrifice, God reveals and defends His justice in two ways. First, Jesus' suffering for the sins of His people means that any sins unpunished beforehand are now fully punished in Christ. God leaves no sin unpunished. Mercy and grace do not come at the expense of justice. Second, because the sins of the faithful are fully punished in Jesus, God may justly declare righteous those who have faith in Jesus. That's what it means to be justified in God's sight - to be declared righteous by faith in Jesus. The cross, rightly understood, is God's own answer to any objection that He is unfair to substitute Jesus for the unrighteous."
-Thabiti Anyabwile, The Gospel for Muslims, pp75-76