One of the classic works in Christian literature is Anselm’s Cur Deus Homo (“Why did God become Man?”). Here, the great teacher of the church works through the problem of sin, coming to the conclusion that it is impossible for man to redeem himself and were man to be redeemed, only one who needs no redeeming could redeem others.
The Heidelberg Catechism tackles this same matter in two questions — why does the mediator need to be man and why does he also need to be God? And, much like Anselm, the framers of the Catechism argue that since it was man who sinned, it is only proper and fitting that man make satisfaction for his sins. Yet, man cannot do that because he is tainted with sin. So, we are in a fix. Only God is without sin of his own and thus only God is able to sinlessly represent man and only God has the power to satisfy the demands of the Law for the elect. Thus, God took on flesh and came as both fully-God and fully-man (except for sin) to make satisfaction for his people.
Sometimes people object, suggesting that God could simply have pardoned the people of their sins as he pardoned the Israelites after their making of the golden calf. Then again, the only reason God pardoned the Israelites is because they were “in Moses” (see 1 Corinthians 10:2). In other words, Moses was their mediator in this matter and Moses serves as a foreshadow of the coming Christ. Further, Moses’ mediation extends only to the earthly consequences of the Israelites’ sin, not to eternal consequences. An eternal man is the only one who has the power to satisfy the eternal consequences of the Law.