I would like to elaborate on this “reason”, as it seems to me it summarizes well the interesting discussion we had in this thread so far:
The key point is St. Thomas Aquinas’ tenet that the state of “original sin” consists mainly in that, after the first human sin, God deprives humankind of those favors that He had accorded to “Adam” in the state of primitive innocence, when God placed him in paradise. Thus, this state was caused by the first human sin, and thenceforth became transmitted to all new human persons coming into existence. This leads Thomas to state that the first sin damaged the whole human nature, which since remained in a fallen state.
This view originates actually from the Greek Father of the Church St. Irenaeus (c.130-c. 202), who in The Demonstration of the Apostolic Preaching states:
11.[…] Moreover he [man] was free and self-controlled, being made by God for this end, that he might rule all those things that were upon the earth. […]
15.[…] if he should keep the commandment of God, he should ever remain such as he was, that is to say, immortal; but, if he should not keep it, he should become mortal and be dissolved to earth from whence his formation had been taken. […]
This teaching was adopted by the Council of Trent (1545-1563), in the Decree on Original Sin, Session 5, Canon 1:
The first man, Adam, when he acted against God’s command in paradise, immediately lost that holiness and justice in which he had been created
And informs today’s Catechism of the Catholic Church:
376. […] the harmony between the first couple and all creation, comprised the state called “original justice”.
404. […] It is a sin which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice. And that is why original sin is called “sin” only in an analogical sense: it is a sin “contracted” and not “committed” - a state and not an act.
So, explaining the “transmission of original sin” amounts to explain why after the first sin God deprives each human person coming into existence of the original holiness God bestowed on “Adam and Eve”.
My proposed answer is that this is the case because God ardently wants to redeem the sinners and to this aim:
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God lets the sinners on earth to give them opportunity to repent, ask forgiveness, and come again to loving God.
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To facilitate the redemption of all sinners God deprives of original holiness all human persons coming into existence after the first human sin, according to Romans 11:32: “For God has bound everyone over to disobedience so that he may have mercy on them all.”
Point 2) has been admirably expressed by Michelle Ols in this post:
Taking all this into account we are led to the following view of what “human nature” is all about:
All humans are bound by God’s love and mercy into “human kind in the image of God”. And this means that you cannot define humanity without reference to God’s mercy; it is God’s mercy which makes that all human persons are related to each other. To the end that, for the sake of redemption, God is led in his mercy to deprive of original holiness and justice all human persons coming into existence after the first sin. In this sense any sin would have provoked the “state of original sin”, if it had been the first human sin.
This makes clear that the views of the state of “original sin” as
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“sin of nature” (Thomas Aquinas, Irenaeus),
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“relational damage” (Ratzinger/Benedict XVI),
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“spiritual pollution resulting from federal headship” (Homo divinus).
are all stating the same with different words.
Notice also that by depriving us of original holiness after the first sin, God let us in a state where our spiritual capabilities are submitted to the strong Darwinian tendencies governing animal life. But, interestingly enough, thereby these tendencies to “red in claw and tooth” themselves become a means to help us humans to love each other, according to the splendid statement by Richard Dawkins: “[…] one of the reasons for learning about Darwinian evolution is as an object lesson in how not to set up our values and social lives. […] We should not live by Darwinian principles.”
In summary, I think the view I am proposing as a result of the amazing discussion in this thread may help:
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To unify the Evangelical, Eastern Orthodox, and Catholic views.
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To better understanding why the “transmission of original sin” remains a corner stone of Christian Belief.
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To show that even evolutionary science may contribute to this understanding.