The notion of atonement is wrapped up with the notion of Adam and Original Sin. You yourself brought the topic to discussion in post #446
The initial reason that Jesus is incarnate, that is He assumed human nature, is that Adam, a human, could not atone or make amends on the divine level of God. Jesus was one person with two natures, one divine the other human. Therefore, He could in justice atone for Original Sin. There are other descriptive reasons for the human nature assumed by Jesus; however, the necessity of “True Man and True God” is the basic one.
From the beginning, St. Paul found a very strong connection between Adam’s Original Sin and Christ’s salvific mission for all people, then and now. (Romans 5: 12-2; 1 Corinthians 15: 21-22; 1 Corinthians 15: 45-49) This need for sovereign justice also follows from the fact that all humankind was in Adam “as one body of one man”.
Where I see that it ties into Adam as a logical proposition is:
Through the original sin, Adam’s physical nature was changed and therefore inheritable to his offspring as proposed by St.Thomas Aquinas in De Malo.
Briefly, one problem with that notion is why original sin is transferable but other sin is not. All offenses are against God, what differentiates, original as transferable. It isn’t Catholic teaching that wicked people have wicked children.
Jesus is seen as “new Adam.” Jesus’ sacrifice is seen to atone for that original sin by Adam. By dying He is said to restore the damage caused by original sin. Anselm said it was honor that was offended, Aquinas said it was God’s justice.
If God is atoning to himself and He possesses infinite power, the atonement should be complete. Or perhaps God’s intention was for us to fall and have our finite natures. The fall was the means to that end. But if we are to believe that God is all powerful and only a God/Man could atone, the atonement wouldn’t be halfway.
The more logical explanation to me, at least, is that biologically we are animals just like all other animals and our nature is that of every other animal. We are mortal as our protohuman ancestors were. Biologically that makes logical sense but not necessarily theologically, but I think a case can be made. Theologically we need something to be saved from in order for a savior to make sense. Adam gives us that reason, as the first man he is the first sinner. Eve actually, but Adam gets the focus.
We inherit the effects of his sin. The privation of the state of grace that Adam lived in. Jesus is said to restore that state of grace in that we have access to it after our death, where Adam had it while living. How is restored? By atoning for the sin of Adam. This is why it comes into the picture. To whom does Jesus atone? It was an offense against God that Adam was guilty of, so the atonement was to God. We have Jesus as God, atoning to God. It’s a closed system.
The implication of God atoning to God are either that God is subject to the system, meaning God has to play with in a certain set of perimeters. Which is a limit to His power, meaning his power is limited not omnipotent. This isn’t Catholic teaching.
Or we can say that Jesus is lesser than the Father. Which Jesus does say (John 14:28). Which carries other theological implications, that the Trinty isn’t equally God in it’s three personhoods, Jesus with his human nature is beneath the Father. Which isn’t Catholic teaching.
There are other theological explanations to explore but the more logical explanation is we have two conflicting desires biologically, as social animals we have sometimes conflicting desires. Some are socially driven that benefit the group and desires for the self, that might not be beneficial to the group. Jesus shows us through His example of self sacrifice that the benefit of the group supersedes the desire of the individual.
Adam isn’t necessary to explain our mortal nature nor that the needs of the group supersede the needs of the individual. So I don’t see Adam as logical necessity. You can still keep the notion of “original sin” in that at some point someone must have sinned first. That is what the catechism seems to imply.
390 The account of the fall in Genesis 3 uses figurative language, but affirms a primeval event, a deed that took place at the beginning of the history of man.264 Revelation gives us the certainty of faith that the whole of human history is marked by the original fault freely committed by our first parents.265
The original sin, is the sin that we are all still subject to. Selfishness.