In Romans 5:14, Paul tells us what he is about: Adam is a type of the coming one, i.e. Christ. A type, by its nature, is a metaphorical comparison, so we cannot demand exact one-to-one correspondence in every detail. That’s not how metaphors work, either in the Bible or in life. For example, take the metaphor of God as “Father.” God obviously has fatherly characteristics, but we cannot extend the metaphor into a literal claim without destroying its meaning or else destroying our concept of God. “Father” may be an appropriate metaphor for God, but it is not the only metaphor, nor is it an exhaustive description of him. (The metaphor “God is love” suffers from the same limitations. Push the metaphor too far, and God is robbed of personhood.) A metaphor reveals similarities between the objects of comparison – in this case, God and a human father – but, like all metaphors, there is never an exact fit in the details, nor should we demand one.
Similarly, when pushed too far, Paul’s metaphorical comparison of Adam and Christ inevitably breaks down. In v. 15, immediately after drawing the comparison, Paul even expressly states that the symmetry isn’t perfect (“it is not the case…”). This imperfect correspondence crops up again in v. 18 when Paul contrasts Adam’s disobedience to Christ’s obedience, where the “one transgression” is paralleled with the “one righteous act” (presumably picking up the thought Paul abandoned at the end of v. 12). Interpreters make the obvious inference that the “one righteous act” is most likely Christ’s obedience in dying on the cross, but the inference by no means resolves the difficulties. We cannot doubt that all of Christ’s life was an act of obedience to the Father. Did none of his prior obedience matter? What if Jesus had sinned before coming to the cross? If Jesus were not perfectly obedient during his entire life, would his one righteous act of dying on the cross still wipe away the damage caused by the “parallel” one transgression of Adam?
In sum, Paul in Rom. 5:12-21 explicitly portrays Adam as a type of Christ. We therefore should not interpret the passage woodenly, insisting upon a perfect one-to-one symmetry in every detail. When we push the metaphor too far, it breaks down and leaves us with the same unresolved situation that has puzzled generations. Instead, we should keep in mind the flow of Paul’s thought in Romans, and how Romans 5 fits within that flow. Paul’s concern is not to affirm Adam as a literal individual. He has other, bigger fish to fry. He chose his metaphor to emphasize one thing: the justification secured by Christ in contrast to the condemnation introduced by Adam, a thought that echoes the central theme of Romans to that point in the letter — justification is available “for all who believe” (3:22).