The Exodus Motif
As I've averred previously, I think the Exodus from Egypt should be the controlling metaphor utilized to understand the atonement. I'm unhappy with the use of the word "metaphor" here, though, as if this was just some sort of Wittgensteinian language-game, disconnected from actual reality. To speak of the atonement in terms of Exodus is to engage in a sacramental-iconic reading of the Scriptures, in which the events of the Exodus are truly connected with the events of the Incarnation, without the one being reduced to the other. The Exodus truly occurred, as did the Incarnation, yet both are sacramentally the same event. I find this way of reading (and inhabiting) the texts extremely revelatory, but also almost impossible to describe in this medium. When I use the word "metaphor," then, it is to be read in a "recapitulatory" manner: Christ is fulfilling the form of the Exodus in His salvific life and death.
The motif itself precedes its explication in the book of Exodus, a sort of adumbration of events to come (just as the Incarnation was foretold in types before it occured historically). We see it in the enslavement of Adam and Eve to the domain of death, for example. However the slavery comes about (whether retribution based on Joseph's enslavement of all Egyptians or as a consequence of distorted willing), the Lord has come to pass judgment on the captor and lead His people out through a trial of water. The connections to justification (understood in a liberative sense: being freed from unjust captivity) and baptism (the Red Sea) are apparent. The strength of this model, other than it being eminently biblical, is that it properly places the wrath of God against the captors, instead of those captive.
The Wrath of the Liberator
One of the commonplaces of, at least, Reformed preaching is that because of sin, God is angry at every human person and only the sacrificial death of His Son can placate that anger. What has made God so angry with all humanity is the primordial (or original) sin: Adam sinned and all humankind bears his guilt, so all bear the wrath of God for that sin. This understanding of the human predicament (whether it comes from Augustine, Anselm, or elsewhere) leads to some very sticky pastoral questions: what about those who never hear of the Christian God? What about babies who are aborted or still-born? (Apparently according to Augustine, prebaptismal deaths necessarily lead one to hell: I cannot verify this reading of the saint, but it is carted out often enough in debates). While maybe God had a beef with Adam, the first man's judgment and punishment are recorded in Genesis 3: he is exiled from the Garden and given over to the forces of entropy and corruption. Why are his children, all of us, then punished for his sin? There is no court of law that would uphold such a practice, yet this is the common interpretation placed on the event and its aftermath. Indeed, here Romans 1:18-32 is brought forth to cement this interpretation: "for the wrath of God is revealed against all godlessness and injustice of men..." There are certain reasons why this passage, though, should not be universalized: have all known God and then refused to be grateful (v. 21)? Have all professing to be wise (v. 22)? Have all dishonored their bodies with "unnatural" uses of the same sex (v. 26)? The answer to these questions are an uniequivocal 'no.' The passage does read well, though, as a retelling of the story from Adam and Eve (who did know God, yet did not glorify Him, nor were thankful -- they also ate from the Tree that they believed would make them wise -- Gen. 3:6) to Sodom and Gomorrah (the exchange of the "natural" use) and so on. It does not make sense to jump from these narratives to a universalized willing depravity. Rather, what St Paul seems to be doing is to take a commonplace of Jewish theology and turning it back against those who would condemn Gentiles while relying on their own religious heritage (2:1, 17, etc.), particularly the gift of the Torah which has paradoxically brought both "Jew and Gentile under sin" (3:9). The Law, then, has revealed the wrath of God against sin itself, but is powerless to stop that force. Humankind is, then, not guilty of Adam's sin and so under divine censure, but is under the power of sin from which it needs to be liberated. The Law cannot do it, especially since it divides Jew from Greek. The point of the Law, then, becomes eschatological: it was intended to create the conditions for God's theanthropic Messiah to come and be put to death by, thus freeing God's humanity (the Jew first and also the Greek) from the tyranny that Adam had put them under. In other words, God's wrath is directed against godless and injust actions (Rom. 1) because it is primarily directed against sin, death, and the devil (7:13, for example).
While We Were Yet Sinners
This context of Exodus liberation (sin/death/Satan playing the part of Pharaoh/Egypt) makes sense of a passage that PSA is necessarily ill at ease with in Romans: "God demonstrates His own love towards us, in that while we were still sinners, Christ died for us. Much more then, having now been justified [set free from our captivity to sin] by His blood, we shall be saved from wrath [the wrath intended for sin/death/Satan] through Him. For if when we were enemies we were reconciled to God through the death of His Son, much more, having been reconciled, we shall be saved by His life" (5:8-10). This brings out an important point: while we were in slavery to sin and the corruption of the world, we were in fact under God's wrath, just as citizens of a country are liable to feel the brunt of a war upon their soil. The Israelites, to go back to Exodus, had to suffer through some of the plagues along with the Egyptians: for those who turned to God, they were an eschatological sign of impending judgment against the evil powers; for those who did not, they became a foretaste of hell. The mercy extended in them, however, was the same from God's point of view: God's judgment against sin is the clarion call to us to "come out from among them and be holy" (2 Cor 6:17). The anger expressed, then, is not towards men in their Adamic guilt, but towards the power of sin over them ("Jew and Greek under sin"): if they remain in that state, however, they feel the consequences of that wrath. Here the will does play a part in the salvation of humankind; it is enslaved to sin, yet the Liberator has come and is calling all men to freedom, even offering all that is needed to make the escape: how shall we respond? (It probably goes without saying here that an enslaved will is not necessarily an inoperative or powerless one).
Baptism and Death
Just as the Israelites needed to have the Egyptians cut off from them by water, so do we need the old impulses of sin and corruption to be washed from us: enter baptism. This requires baptism is be more than promissory or merely symbolic: it must be sacramental. This is in line with St Paul's argument concerning baptism in Romans 6. We who have been baptized have shared in the actual death of Christ, dying along with Him, sharing in the final Exodus moment of freedom. Sin and corruption have no rightful claim over us, therefore, and we need not heed them. (This also helps to make sense of the debates between communions as to whether the various baptisms are valid or not.) This then connects the Exodus back to the Garden: to be free from the power under which Adam foolishly (God forgive him) put us, death was required. Baptism is our death into Christ, He who death has no rightful claim on, so our participation in Him through faith means that we can, in the present time, rise to "newness of life" (6:4) and, in the age to come, experience the adoption, "the redemption of our bodies" (8:23). Christ's death, then, is necessary not only as the "demonstration of God's love" (5:8), but also as the event which judges sin and Satan, finds them wanting, and destroys their power of death (Heb. 2:14, cf. John 16:8-11). As noted, this is an eschatological reality: what has happened in Christ is complete, but not fully realized under the Last Day -- Satan has been judged and cast out (exorcised), but yet he still prowls around like a lion. The difference is in whether we are "in Christ" or "in Adam."
The Continuing Wrath?
What do we make, then, of the Wilderness Wanderings? Don't we see God's wrath poured out, over and over again, on His people then? Let us remember that these things are written to us as types, for our admonition (1 Cor 10:11). In other words, God's judgment on His people in the wilderness (and when they were in the Promised Land) is meant to cleanse out the old Egyptian life, just as our discipline from God (which can include our physical death -- 1 Cor 11:30) is meant to purge out the consequences of Adam's sin (death and the corruption of our nature, not guilt for his action), so that we might be "conformed to the image of His Son" (Rom. 8:29), which was the plan God had from the beginning. The Wanderings should be read as a narrative of "making good on baptism": as truly dying to the world through repentance, so that we might live. Christ is recapitulating these events in us so that we might be fully made holy in His love.
This is, obvious, just a bare-bones sketch of an atonement theory: I haven't dealt with the sacrificial system in any way here. Much more could and, of course, should be said: about the importance of the Incarnation and hypostatic union, our becoming "co-workers" with Christ in the Christian life, and so on. It is meant as no more than a different, yet biblically coherent, reading of atonement as set against Penal Substitionary Atonement theory. I'm sure I've made mistakes here; God forgive me.