It begins:
If poisonous minerals, and if that tree,
Whose fruit threw death on else immortal us,
If lecherous goats, if serpents envious
Cannot be damned; alas, why should I be?
The doctrine on original sin holds that after the fall, all of creation was corrupted--the sin of humankind brought death into creation for all. Yet only humans in earthly creation are subject to eternal punishment. The poet questions this: there are harmful, bad, "evil" elements in nature that are not subject to the threat of damnation. The poet's challenge may go further: instead of humankind's sin bringing death to nature, perhaps nature (in the form of the tree) is responsible for bringing death to humans.
But inherent in this challenge is an evasion of responsibility. The poet thrusts the blame for the fall on the tree and its fruit. In the Genesis story, Adam also tries to evade responsibility, answering God, "The woman whom thou gavest to be with me, she gave me of the tree, and I did eat." Adam shifts blame to Eve, and to God. Eve's response is much simpler: "The serpent beguiled me, and I did eat." It's still an evasion (blaming the serpent), but takes more responsibility than Adam (Milton handles this beautifully in Paradise Lost, giving Adam a lengthy speech, and giving Eve her simple sentence).
The next challenge springs from the first:
Why should intent or reason, born in me,
Make sins, else equal, in me more heinous?
The poet answers the previous question: the reason humans must bear responsibility for their "sins" (while rocks, plants, and animals bear no responsibility) is because of reason and intent. Animals can't reason, thus can't sin--their actions lack intent.
The poet asks why reason and intent should make human actions damnable as opposed to the actions of rocks, plants, and animals, but I think today the challenge goes further. I can't help but think of two 19th century titans.
Darwin rips apart the distinct separation between humans and animals--we are a part of nature, subject to and emergent from the same evolutionary forces of the rest of nature. Even our reason is but an evolutionary development. Like every other living thing in nature, we struggle for survival. So if we are not created fundamentally different from other parts of nature, why are we held accountable in a way other parts of nature are not?
And in Notes from the Underground, Dostoevsky rips to shreds the idea that humans behave according to reason, suggesting it is irrational desires that have motivated human behavior all through history. If humans are irrational, then what is the "reason" that would hold humans accountable for sin? We behave according to all sorts of unreasonableness, sometimes against our own interests. Why, then, sin?
Finally, the poet questions God's mercy:
And mercy being easy, and glorious
To God, in his stern wrath, why threatens he?
If God is loving and merciful, then why does he threaten humans with damnation? Couldn't he just show mercy? Can't he--and wouldn't he want to--forgive humans without requirement? It is a common--and powerful--question about the nature of the Christian God.
In the Petrarchan structure of this poem, the octet sets out the problem. The poet asks three questions about God that come down to this: why is there hell at all? The problem is posed: how will the sestet resolve the problem?
But who am I, that dare dispute with thee
O God?
What? The poet sets out serious challenges to Christian doctrine on damnation, calls into question the threat faced by humans, even directly challenges God's mercy...and then backs off in humility?
This means, of course, that the questions he raised in the octet are unresolved. Even if he is now abandoning the questions, recognizing the superiority of God over himself, he's still leaving the questions. The reader is left with them, deliberately unanswered--because there are no satisfactory answers.
The rest of the poem breaks off into conventionality:
Oh! of thine only worthy blood,
And my tears, make a heavenly lethean flood,
And drown in it my sin's black memory;
That thou remember them, some claim as debt,
I think it mercy, if thou wilt forget.
I find this mostly forgettable, except perhaps the last line. The word "mercy" will call to mind the use of "mercy" in lines 7-8. The poet asks God for the mercy that God supposedly has but doesn't seem to show. Does the last plea for mercy subtly continue the challenge? Perhaps.
What one is left with in this poem are the three questions in the octet. I can't think the humble rejection of the questions in line 9 is entirely sincere; the questions have serious theological consequences, and are difficult to leave behind in a conventional show of humility and repentance. The poem asks major theological questions about sin and damnation. But perhaps the ideas are even more fundamental: the poem asks questions about humankind's place in the cosmos and about the nature of God.