Donne's hymn opens like a cross-examination in which the accused is the one asking the questions. "Wilt thou forgive that sin where I begun?" he asks God, and the question comes back in each of the first two stanzas, attached to a new confession. First is the sin he was born into, "which is my sin, though it were done before." That seems to allude to inherited guilt, yet the speaker insists on owning it. Then comes habit: "through which I run, / And do run still, though still I do deplore." The doubled "still" catches the trap neatly, since he repents and repeats in the same breath. In the second stanza the damage spreads outward. He has "won / Others to sin" and made his sin "their door." Worst of all is the failed reform, a sin shunned "a year or two, but wallowed in a score." "Wallowed" is blunt and physical, and "score" sounds like a tally, as if his repentance were a ledger of failures.
The refrain does the real work: "When thou hast done, thou hast not done, / For I have more." Here "done" means "finished forgiving," but it is hard not to hear the poet's own name in it. Some readers also catch an echo of his wife's maiden name in "more," though that is a reading rather than something the poem states. Either way, the wordplay is not a joke at prayer's expense. It is how he bargains. He turns the language of finishing and exceeding against God's own generosity and asks whether mercy can ever run out.
The form enacts this circling. Every stanza follows the pattern ABABAB, and the whole poem uses only two rhyme sounds. One is "-un" (begun, run, done, won, shun, spun, Son), the other "-ore" (before, deplore, more, door, score, shore, heretofore). Because the rhymes keep returning to the same sounds, the poem seems unable to leave its own problem. The lines are mostly ten syllables long, but the refrain shrinks to eight and then four ("For I have more"). That sudden drop reads like an admission, a voice trailing off. The diction supports it, since the language is almost entirely plain, one-syllable words, closer to a song or a spoken prayer than to a display of learning. That plainness suits the title.
The last stanza turns the poem. The speaker finally names his deepest sin, and it is not any of the earlier ones: "I have a sin of fear." The imagery moves from guilt to danger. Life is a thread being spun, and when the "last thread" is finished he fears he will "perish on the shore," like a sailor wrecked in sight of land. What he dreads is not that he has sinned but that mercy might fail him at the last moment. So he asks for something bold: "Swear by thyself." An oath on God's own name is the strongest guarantee imaginable, because nothing higher is left to swear by. What he wants sworn is that "thy Son / Shall shine as he shines now and heretofore." The pun on Son and sun is the poem's most beautiful one, and it gives the promise a physical image of light that does not dim.
The ending reuses the opening's words with their meanings reversed. "Done" now means that God's work of reassurance is complete, not that forgiveness is used up. "More" turns from "I have more" sins to "I fear no more." The same rhyme words that trapped him in the first two stanzas now release him.
The poem matters because it joins wit and sincerity so closely that you cannot pull them apart. Donne, who was a preacher as well as a poet, makes theology feel like a private, nervous conversation. The central idea is surprising and psychologically sharp. Piling up sins is not the real danger. The real danger is doubting that forgiveness is big enough to cover them. A poem that begins in anxious accounting ends in trust, and its puns carry that change.