The holiness of God becomes our salvation not by slackness of demand but by completeness of judgement; not because he relaxes his demand, not because he spends less condemnation on sin, lets us off or lets sin off, or lets Christ off ('spared not'); but because in Christ judgement becomes finished and final, because none but a holy Christ could spread sin out in all its sinfulness for thorough judgement.
I have a way of putting it which startles some of my friends. The last judgement is past. It took place on Christ's cross. What we talk about as the last judgement is simply the working out of Christ's cross in detail. The final judgement, the absolute judgement, the crucial judgement for the race took place in principle on the Cross of Christ. Sin had been judged finally there. All judgement is given to the Son in virtue of his Cross. All other debts are bought up there.
From The Work of Christ, by P T Forsyth, page 137