I'm really enjoying John Stott's book The Cross of Christ. One of the things I like about it is that because he comes from a different generation than the current popular Christian writers, he refers to other commentators and writers which I may be unfamiliar with. In his chapter called "The Self-Substitution of God," he quotes C.E.B. Cranfield, who made a succinct statement about the substution of God:
God, because in his mercy he willed to forgive sinful men, and being truly merciful, willed to forgive them righteously, that is, without any way condoning their sin, purposed to direct against his very own self in the person of Son the full weight of that righteous wrath which they deserved.
That's a reality to rejoice in.