Substitutionary Atonement

From a pastor-friend:

I was hoping you or someone (where are you, Spurgeon?) could give us a reasonable defence, or at least clarify the parameters or valid aspects of the substitutionary theory.

I know for example that the idea that 'the Father turns his face away' [quote S.Townend 'How Deep the Father's Love'] from the Son at the moment on the cross in which Jesus cries Eloi Eloi lama ...because at that point the Son is bearing the sin of the world and the holy God is thus forced to separate from him -- this doctrine is not explicitly taught in Scripture and has grave problems as a theory. [Can Father and Son separate? Is Jesus abandoned at point of greatest need? How does he acquire the sin of the world? Why is he trustful ('Into your hands...') a few moments later - has the sin been paid for now? Is he merely quoting Ps 22, conscious that as that psalm turns to victory, so will this abandonment?]

The idea also that 'a holy God cannot live with anything sinful and unrequited' has problems because we teach that God's Spirit indwells us tho we are clearly sinful, so why the demands of payment for sin that are fundamental to the forensic theories?

I feel that Nathan's discussion will win the day in the minds of most of the readers (- a radical departure from traditional evangelical teaching) if no one can explain the merits of the idea of vicarious substitutionary propitiation for us, or at least point to worthwhile discussions in textbooks. I recall that Leon Morris and C.H.Dodd went head-to-head over this years ago, and I think the consensus was that Leon won on points arguing for the conservative view, which upset the comfortable English liberal schools. Perhaps I should check that out and post the salient points.