Donne opens with a line that sounds like a boast: "I am a little world made cunningly." In his day, "cunningly" meant skilfully, so the speaker is first of all a masterpiece, a miniature universe built from "elements, and an angelic sprite", body and soul together. The pride lasts about a line and a half. By line three "black sin hath betrayed" this fine creation "to endless night", and the verdict arrives in a blunt, breathless line: "My worlds both parts, and (oh!) both parts must die." That parenthetical "(oh!)" is the poem in miniature. A man's private panic interrupts what should be a calm statement of doctrine.
The central idea is that sin has ruined the whole person, and that no ordinary remedy will repair it. Donne then does something startling with the idea of a "little world". Having called himself a world, he treats himself as one, and turns to the people who explore worlds. The "you" in line five is addressed to those who have "found new spheres" beyond the old outermost heaven and "of new lands can write". Judging by the imagery, this means astronomers and voyagers, the people stretching the map of the universe. The speaker wants something from them, though, and it is not information: "Pour new seas in mine eyes." He asks to borrow their discoveries as tear-ducts. The new oceans are to flood his small world with weeping, and the grand language of exploration shrinks to the size of one man's guilty face.
The poem then reasons with itself. If the world cannot be drowned again, then water can only "wash" it. Most readers hear an allusion here to the promise made after Noah's flood that water would not destroy the earth a second time. But water is not enough: "But oh it must be burnt!" Again an exclamation breaks into the pentameter, and the line lurches. Fire, however, is already a corrupted element in this world. "The fire / Of lust and envy" has been at work "heretofore", and has only "made it fouler". The sentence spills across the line break, as if the guilty flames cannot be contained.
The ending turns on this double meaning of fire. The speaker does not ask to escape burning. He asks for a different blaze: "Let their flames retire, / And burn me, O Lord, with a fiery zeal / Of Thee and Thy house." "Retire" is a military word, so the fires of lust and envy become an army sent off the field. The last line, "which doth in eating heal", echoes a biblical phrase about zeal for God's house eating a person up. The paradox is the point. Consuming and curing are the same act, and the only way to be saved from burning is to be burnt by something holier.
The form carries the same tension. The poem is in iambic pentameter, and the first line runs smoothly, but exclamations and parentheses keep roughening it, so it sounds like someone thinking aloud under pressure. The octave rhymes abba abba, in the Petrarchan way, though "cunningly" and "earnestly" have to rhyme with "die" and "high". That forces the light ending of those words to ring out, and the pride of "cunningly" is dragged into the same sound as death. The sestet rhymes cdcd and closes on a couplet, "zeal" and "heal", which lands with the confidence of an answer after all the questioning.
The poem matters because it takes the huge, public language of the new astronomy and exploration and turns it inward. A whole universe of discoveries ends up as a metaphor for one conscience. It is also bold in how it treats its own images. Water, fire, world, and sin are each used, doubted, and revised until the poem finds the combination it can trust. The result is a prayer that argues with itself, and part of its force is that the argument is so clearly alive.