A simpler and more familiar solution for the problem of how God can be just and still justify the unjust is found in the Christian doctrine of redemption. It is that, through the work of Christ in atonement, justice is not violated but satisfied when God spares a sinner. Redemptive theology teaches that mercy does not become effective toward a man until justice has done its work. The just penalty for sin was exacted when Christ our Substitute died for us on the cross.
– A.W.Tozer; Knowledge of the Holy; Kindle Version; Page 70-71