It's sometimes said that the concept of "honor" with which Anselm works is rooted in medieval feudalism and that, outside this context, this model of the atonement therefore falls apart. It's important to note that, when Anselm talks of the honor God is due, he is talking about the worship that the creature owes the Creator.
Anyways, if you're going to reject or critique Anselm's account, even in passing, you should at least have an accurate view of what he argues, and understand him on his own terms. So, in what follows I take a quick stab at laying out his argument for you, with references to the pertinent sections of the two books. I've skipped over some things (e.g., the discussion of why the number of fallen humanity couldn't be restored by angels), and I've left out any biblical references. This is designed to function as a rough-and-ready guide for you to read Cur Deus homo? for yourself.
Without further ado, here's my synthetic outline of Cur Deus homo?
A. The redeemer must be a divine person (1.5)
- The one by whom humanity is redeemed is owed worship and service.
- Humanity is not to worship and serve anything other than God.
- Therefore, the one who redeems humanity must be God.
B. The redemption through this divine person cannot be externally compelled. (1.6-1.7)
- If God is compelled to redemption by, e.g., the devil's power over human beings, then he is not omnipotent.
- But God is omnipotent, and all things are in the power of his will.
- Therefore, redemption must be compelled by God's will alone.
C. The mode of this redemption (i.e., the incarnation of Christ the Son) must be necessary, consequent upon the wisdom of the divine will. (1.6-1.7)
- God is supremely wise, which means he doesn't do anything unnecessarily.
- God willed to redeem humankind through the incarnation of his Son.
- Therefore, the incarnation of God's Son must be necessary for redemption.
D. The mode of this redemption (i.e., the incarnation of Christ the Son) must be voluntary, consequent upon the justice of the divine will (1.8)
- God is just, which means he does not force the innocent to bear the sins of the guilty.
- Redemption came about through the incarnation of Christ the Son.
- Therefore, the incarnation of Christ the Son is not a case of the innocent being forced to bear the sins of the guilty.
- What God commanded Christ the Son was what he commands of humankind generally: to live a righteous life of worship (or honor) of God.
- This was the command that Christ the Son obeyed, and it was for obedience to this command that sinful humanity killed him.
- Therefore, Christ the Son's death is only secondarily related to redemption, consequent upon the state of those he came to redeem.
- What is owed God is the "honor" he is due as Creator, that is, total and complete worship.
- There is nothing greater than this that can be given to God in exchange for failure in this regard.
- Therefore, this requirement of total honor must be satisfied by humanity.
G. The necessity of punishment for failure to honor God is consequent upon the perfection of the divine nature. (1.13-15)
- God's honor is incorruptible and unalterable, and every creature owes God this honor by virtue of its being related to him as Creator.
- The presence in God's creation of dishonor is an ontological challenge to God's very nature.
- Therefore, on the pain of compromising the perfection of the divine nature (impossible), God must punish sin.
H. The punishment due for sin is proportional to the honor not given to God. (1.19-24)
- The honor that God is due, by virtue of his nature, is infinite and total.
- The punishment given for failure in this regard must be proportional to this failure.
- Therefore, the punishment humanity incurs necessarily (on pain of God forfeiting his nature, which is impossible) is infinite and total.
I. Only Jesus Christ as the divine-human mediator can render to God the positive honor he is due, and also bear the negative consequences of humanity's failure to honor God. (1.25)
- To offer the honor God is due, he must be fully human. (2.6)
- To overcome the infinite punishment humanity has incurred, he must enter into it as one whose infinite nature can overcome even this. He must be God, who is greater than all things. (2.7, 14-15)
- Therefore, he must be both human and God, with these two natures neither mixed together, nor separated, but united hypostatically, in one person. (2.8-9)
- God is omnipotent and his purposes cannot be frustrated.
- God wills to create a humanity that honors him and attains blessedness.
- Therefore, God must necessarily become incarnate and redeem humanity when it falls.
What follows from all of this is that redemption, while it is achieved by Christ, is entered into only through being joined to Christ through the Spirit. Participation in Christ as the one who obeys and the one who undoes the consequences of sin ("punishment") is indispensable.
When it's all said and done, Anselm's interlocutor in this work, Boso, concludes of this model of the atonement: "There can be nothing more logical, nothing sweeter, nothing more desirable that the world can hear. I indeed derive such confidence from this that I cannot now express in words with what joy my heart is rejoicing" (2.19). You may not come to the same conclusion, but I hope at least you'll achieve a more accurate understanding of this great text.