“If Jesus didn’t die in order to overcome humanity’s original sin, then why did Jesus die? What is Jesus, the second Adam, attempting to restore with the cross, if not the sin of the first Adam?”
Here’s my two cents:
I believe substitutionary atonement is one of the models of atonement theory that unquestionably belongs in orthodox Christian theology. Paul and other biblical writers clearly and repeatedly represent Jesus’ death as being substitutionary (He died for us and in our place) and as taking our punishment. Alone, the substitutionary model does not describe everything the cross and resurrection accomplished and continues to accomplish, which is why we have other models and metaphors to flesh out the fullness of what the atonement means. (Christus Victor where Christ conquers sin, death, and the power of evil, the representative model, where Christ is the faithful Israelite and succeeds where every other human has failed, the judicial/legal model where our debts are paid or our crimes are acquitted, the ransom/rescue metaphor where we are rescued from the Kingdom of darkness and brought over to the Kingdom of light, etc.)
It seems you are presenting a false choice. I hear you saying, if there was no historical fall or historical Adam, than the whole substitutionary atonement model falls apart. Not necessarily. That is only the case if the only point of the cross is to deal with original sin. “Original sin” is a theological construct that can be understood in a variety of ways, and furthermore, what church is teaching that the point of the cross is to erase Adam’s sin, or put another way, erase humanity’s sinful nature? Clearly, the cross didn’t restore humanity to a state of actual sinlessness, at least not in the present.
I have appreciated reading Scot McKnight and Peter Enns on this subject, particularly their exploration of the “image of God” or Eikons as represented throughout redemptive history, and their understanding of sin as rebellion against God’s rightful rule. For example in The King Jesus Gospel, McKnight traces the history of failed Eikons from Adam and Eve (which you don’t have to understand as the literal genetic father and mother of all humanity, you can take them to be representative of God’s original covenant with humanity) to Abraham, to the people of Israel, to the kings of Israel. The story of redemption is the story of Christ as the faithful image bearer of God, whose death and resurrection make possible the faithful image bearing of the Church, a people brought out of rebellion and into the Kingdom, submitted to Christ’s faithful rule as God’s ultimate Eikon.
What Jesus restored is the possibility for right relationship with God and submission to his rule, not sinlessness. The rebellion that Adam and Eve represent and all humans participate in is treachery and you cannot build a Kingdom of traitors. We all deserve to die for our treason. Not because God is cruel, but because he is the just and rightful ruler of his own world and he cannot tolerate treason and effectively rule the world he created. The cross makes a way for us to belong to God’s people and be faithful subjects of the Messianic King to whom God has granted all authority, because it vicariously puts to death the treason in our hearts.
I don’t think the Cross makes sense outside of a larger eschatological picture that encompasses the mission of God on earth. It was God’s plan from eternity to be united with his creation and to live with his people in a New Creation, no matter what you believe about the origins of those people or how the Old Creation got here. Since the beginning, there has been the promise of the New Creation, which has been breaking into our world with the Incarnation, the Cross, the Resurrection, the Ascension, and Pentecost. The goal is not sinlessness (though that will be a beautiful feature of the fully established Kingdom of God on earth) the goal is the undisputed and uncontested reign of God mediated by Christ, his only faithful Eikon.