Don Carson has written a great article on why the doctrine of penal substitution comes so readily under attack [HT: Mike Gilbart-Smith].
I particularly found the section I've quoted below helpful. Talking to some people recently, one does walk away with the impression that certain views of the cross can be freely ignored or dismissed as just conjectures from a particular viewpoint of history. Enjoy:
In recent years there has been a lot of chatter about various “models” of the atonement that have appeared in the history of the church: the penal substitution model, the Christus Victor model, the exemplary model, and so forth. The impression is frequently given that today's Christians are free to pick and choose among these so-called “models.” But for any Christian committed to the final authority of Scripture, this approach is methodologically flawed. It allows historical theology to trump Scripture. Surely the right question to ask is this: Which, if any, of these so-called “models” is exegetically warranted by the Bible itself? For instance, are there passages in which biblical writers insist that Christ in his death triumphed over the powers of darkness? Are there passages in which Christ's self-sacrifice becomes a moral model for his followers? Are there passages in which Christ's death is said to be a propitiation for our sins, i.e. a sacrifice that turns away the wrath of God? If the answer is “Yes” to these three options--and there are still more options I have not mentioned here--then choosing only one of them is being unfaithful to Scripture, for it is too limiting. Christians are not at liberty to pick and choose which of the Bible's teachings are to be treasured.
There is another question that must be asked when people talk about “models” of the atonement. Assuming we can show that several of them are warranted by Scripture itself, the question to ask is this: How, then, do these “models” cohere? Are they merely discrete pearls on a string? Or is there logic and intelligibility to them, established by Scripture itself?
One recent work that loves to emphasize the Christus Victor “model”--Christ by his death is victor over sin and death--somewhat begrudgingly concedes that penal substitution is found in a few texts, not least Romans 8:3. But this work expends no effort to show how these two views of the atonement should be integrated. In other words, the work in question denigrates penal substitution as a sort of minor voice, puffs the preferred “model” of Christus Victor, and attempts no integration. But I think it can be shown (though it would take a very long chapter to do it) that if one begins with the centrality of penal substitution, which is, as we have seen, grounded on a deep understanding of how sin is an offense against God, it is very easy to see how all the other so-called “models” of the atonement are related to it. The way Christ triumphs over sin and death is by becoming a curse for us, by satisfying the just demands of his heavenly Father, thereby silencing the accuser, and rising in triumph in resurrection splendor because sin has done its worst and been defeated by the One who bore its penalty. Moreover, in the light of such immeasurable love, there are inevitably exemplary moral commitments that Christ's followers must undertake. In other words, it is easy to show how various biblical emphases regarding the atonement cohere if one begins with penal substitution. It is very difficult to establish the coherence if one begins anywhere else.