The sermon takes its title from the doctrine of vicarious substitution: the justice of God requires payment for sin, and the whole Old Testament sacrificial system existed to picture and anticipate that payment. Every sacrifice pointed toward a substitute bearing what the guilty deserved, so that God’s righteous requirements against human transgression could be satisfied rather than set aside.
That substitution is what the cross accomplishes for good. The sermon poses the questions directly: why did Jesus have to die on the cross, why did God the Son have to become a human being, and why did He have to partake of flesh and blood at all. The answer running through each question is the same — only a true substitute, fully God and fully man, could stand in the place of sinners and fully satisfy and appease God’s wrath.