Anselm on Sin Necessitating the Atonement

This book goes on to prove by rational necessity - Christ being removed from sight, as if there had never been anything known about him - that no man can possibly be saved without him.  [ . . . ] And I show that all things which we believe about Christ ought, necessarily, to occur. 
- Anselm

In Anselm's work Cue Deus Homo?, we learn that Anselm holds a particularly strong view on the problem of sin.  He argues that we cannot work to remove sin, for any work that we do is something we owe to God anyway.  This truth just follows from the definition of God as the greatest conceivable being and what we owe God as his creatures.  Sin will remain forever no matter how hard we work.  It follows that no matter how small a particular sin is (if it is small) that we are incapable of paying for it.

Anselm uses this conception of sin to set up his doctrine of the atonement.  Given the supposition that God wishes to save some of us, Anselm holds that his conception of sin can show that the atonement is necessary.  And given that Anselm's doctrine of sin can be maintained on grounds of natural theology alone, this project qualifies as an example of the sort of congruence I mentioned in an earlier post about how peculiar Christian doctrines can link up with what we learn from natural theology in surprising ways.  This sort of congruence can provide additional justification for Christian belief.

Here's how Anselm thinks this works:  Those who owe the debt must pay, so the agent that pays the debt must be a human - (here Anselm is drawing on his Realism, which I don't care to explain because I think his use of it is wrongheaded and unnecessary for the cogency of his position.)  Later Christian tradition holds that the agent paying the debt must stand in the correct sort of relation to those receiving the benefit, which is the relation of federal headship.  But Anselm takes it that this federal head must also be God because humans cannot ever give to God something that isn’t already owed to him.  Only a God-man could atone for sin.  This God-man then dies an unjust death, accruing merit.  Jesus offers this bonus to the Father.  The Father is obligated to fulfill what Jesus wants, and Christ wishes for his fellow man to be saved from their sin.  The law is satisfied and mankind escapes punishment through this inter-Trinitarian exchange.  If this argument works, we have a strong reason to think that God must become incarnate and atone for sin if he wishes to pardon us. 

This isn't exactly the view of the atonement that would become popular with the Reformers, but it's a close neighbor.  The Reformed tradition holds that the death of Christ wasn't so much as to accrue excess merit but rather to provide satisfaction for the demands of the law in the form of undergoing punishment on our behalf. The adjustments to make Anselm's argument work for this view of the atonement are slight and don't change anything of substance.