So mankind, as a species, fell into sin, became subject to death, and bound in soul. To redeem this circumstance, both sin (and with it death) and the bondage of the soul have to be undone. The atonement of Christ is God's solution to this dilemma. Throughout church history there has been some confusion about how Christ's atonement actually functions: some have seen it primarily as a penal satisfaction, others the payment of a ransom, still others as a typical example or an illustration. The earliest nonbiblical testimony seems to favor the ransom theory; the Bible itself, I think, casts the efficacy of the atonement in the light of penal substitution. The debate is silly, really, for the atonement actually entails and accomplishes all of these functions.
Although the notion of ransom is well attested in the OT, and spoken of very directly in the NT, I see the penal substitution aspect of the atonement as more vital than any other aspect. If I'm standing in fellowship with the saints before God, but have no clear, final disposition of my past sin; even amongst the throng before the throne, individual doubt would arise within me, and guilt with it. Reflexively my eyes would turn from him who sits on the throne. So redemption apart from penal substitution won't produce what the gospel invites us to-- guiltless, intimate, barrierless fellowship and interaction with the Holy God.
In my mind, the idea of group redemption does no more to salve the individual conscience than the concept of inherited sin does to convict the individual conscience. How can a person truly own either one? Even the sacrificial system of the OT, which pointed to Christ, shows that both communal and individual sacrifice was necessary to alleviate guilt and provide restoration with God. So, though Christ was prototypical in the way that Adam and Eve were, and what he did on the cross delivered a kingdom, it is still its application to the individual that makes the difference. It is the individual who must so identify through faith with Christ's death and resurrection that he or she sees themselves upon the cross, in the tomb, and coming out to new life in Christ.
That is the message of Romans 6. The atonement works for the individual through uniting by faith with Christ in his vacarious sacrifice. The word translated united really carries the meaning of entwined. The atonement works for the individual when he or she sees not Christ on the cross, or buried, or walking alive out of the tomb, but his or herself! Jesus gave himself as the vessel that can be any of us vicariously, that's how the atonement gets out of the realm of theories and does its practical good for me.
For that, I say, "Thank you Jesus!"