An Obligation on the He Who Cannot Be Obliged
To some degree, the theological principle that Bruce Marshall describes here can be seen as a core division point of human ideology:
If God had remitted our sins by sheer forgiveness—sent them away or simply declared them nonexistent—then our sins indeed would be gone, and we no longer would be sinners. We would, however, be mere spectators to our own salvation: observers who simply noted this fact about ourselves, without any involvement of our hearts and wills. By treating our sins as a debt for which he will accept payment, God gives humanity a genuine share in its own salvation. As any child knows whose father has given him or her money to buy him a Christmas gift, there is joy in this that can come in no other way, even though—or, better, precisely because—we know well that we are simply giving back what we have freely received.
Theologically, I’d suggest that the salvific transaction is actually more profound than that. Undeserved blessings are arbitrary and may be removed arbitrarily. God’s granting us an ability earn salvation conversely creates an obligation on Him to provide it — to reward.
Thinking of the myriad people, in modern society, who appear to believe that they are owed happiness and comfort, in material matters, and should face no strings along with spiritual beneficence, it’s difficult to avoid the impression of a paradox: Many are eager to trade that which makes them human — the ability to judge the world and choose a path through it — for creature comforts, yet in so doing, they inflate their importance in the universe.
The image that comes to mind is of an impetuous child who understands that he or she is gong to receive a reward, anyway, and scorns and challenges his or her parents for imposing a chore — a game. The parents are giving the child an opportunity to place a binding claim on them, and the child is insisting that he or she already owns that claim, and more, as payment for deigning to exist.