How is it possible for the righteous God to declare the unrighteous to be righteous without either compromising his righteousness or condoning their unrighteousness? This is our question. God’s answer is the cross.
Without the cross the justification of the unjust would be unjustified, immoral, and therefore impossible. The only reason God ‘justifies the wicked’ (Rom. 4:5) is that ‘Christ died for the wicked’ (Rom 5:6). Because he shed his blood (Rom 3:25) in a sacrificial death for sinners, God is able justly to justify the unjust.
According to the Christian revelation, God’s own great love propitiated his own holy wrath through the gift of his own dear Son, who took our place, bore our sin and died our death. Thus God himself gave himself to save us from himself. This is the righteous basis on which the righteous God can ‘righteous’ the unrighteous without compromising his righteousness.–By John Stott