This "Holy Sonnet" is built on a foundation of religious allusion; this is a poem about what it means to live with Christian faith.
The poem opens with a vision of Judgment Day: the end of the world as foretold in the surreal, prophetic Book of Revelation, which forms the last chapter of the Bible. On this day, Christian tradition holds, the souls of the dead will return to their "scattered bodies," everyone who's ever lived will rise from the grave, and God will judge them.
Besides alluding to Revelation's image of four angels—one for each "corner of the earth"—kicking off the festivities, the speaker evokes the sheer overwhelming scale of this day with a list of all the ways the dead have died: "All whom war, dearth, age, agues, tyrannies, / Despair, law, chance hath slain," he says, will get right back up again.
For that matter, everyone who the "flood did, and fire shall, o'erthrow" will be there—an allusion to traditional biblical stories about world-ending divine judgment:
- The "flood" here refers to Noah's flood, in which God drowns nearly everyone on earth in order to start afresh.
- Afterwards, God assures Noah (who, along with his family, is one of the few righteous survivors of the flood) that God will never destroy the world by water again, a pact known as the "covenant of the rainbow."
- Later Christian tradition holds that God's promise not to drown the world doesn't mean he won't destroy it again: next time, the Apostle Peter warns, he'll just use fire!
- These lines again stress the universality of the Last Judgment. Everyone who died in the ancient Flood will be there; so will everyone who's going to die in the coming apocalyptic fire.
The living—those who will "never taste death's woe"—will be there too, of course! That thought makes the speaker feel rather uneasy. If Judgment Day were to come today, he worries, he'd be among the crowd of the living, waiting to be judged, and he's not so sure he'd come off well. He feels his "sins abound"—that is, that he's got a big heap of sin to atone for. This is why, in his closing lines, he begs God to "teach [him] how to repent" for his bad behavior.
Really, though, he's only half worried about this: his faith assures him that God wants to forgive him. That comes through in his last allusion. When, speaking to God, he declares that learning to repent would be "as good / As if thou hadst sealed my pardon with thy blood," he's reminding readers that Christianity claims God has already done exactly that. The reference here is to the Crucifixion, in which Christ—God in human form—sacrifices himself to atone for humanity's sins.
This act, the speaker believes, shows that God absolutely longs to forgive sinners, and is willing to go to great and terrifying lengths to do so! In merely asking for God's help with repentance, the speaker can trust that God forgives him already.