Most of us lower our voices when we talk about death. Donne's speaker does the opposite. He squares up to it like a playground bully-buster and tells it to stop showing off. The sonnet's central claim is that death's fearsome reputation is a bluff. The poem assumes a Christian belief in resurrection ("we wake eternally"), so death is not an ending but a brief interruption. The last line turns that belief into a paradox: "death, thou shalt die."
The poem is built like a debate speech, and much of its pleasure lies in watching the argument advance. The octave uses the interlocking rhymes of the Petrarchan sonnet (thee, so, overthrow, me), which gives the first eight lines a tight, boxed-in feel, as if death is being cornered. In the first quatrain the speaker denies that death truly kills anyone: "those whom thou think'st thou dost overthrow / Die not." The second quatrain brings in a clever piece of logic. Rest and sleep are only "thy pictures", copies of death, and if the copy gives pleasure, the original should give far more. Even the deaths of "our best men" are recast as a kind of gift, "rest of their bones and soul's delivery", the body resting while the soul is freed.
Line 9 is where the poem turns, and here the speaker moves from argument to insult. Death is demoted from monarch to servant: "slave to fate, chance, kings and desperate men." The line is crowded with heavy monosyllables, like a list of rough company. Death turns out to "dwell" with "poison, war and sickness", lodging with low company rather than ruling from a throne. Worse, it is replaceable. "Poppy or charms can make us sleep as well / And better than thy stroke." The question that follows, "why swell'st thou then?", is the sharpest jab in the poem. To swell is to puff yourself up, and the image makes death look like something inflated with its own self-importance.
The sound of the poem supports this tone. The opening, "Death, be not proud", puts a stress on the very first syllable, so the otherwise regular iambic pentameter lands like a fist on a table. The constant "thee" and "thou" matter too. In Donne's day these were the forms used for addressing inferiors, so the grammar itself talks down to death. The phrase "poor death" does the same job with pity, the most deflating kind of contempt.
The ending is where the whole poem has been heading. A sestet that rhymes cddc is closed by a couplet, an English-sonnet habit, and the final line splits neatly at its semicolon. In the first half, "death" is a noun the speaker talks about: "death shall be no more." In the second half it is a person he talks to, and its name is repeated, then ended by the last word of the poem: "die." The rhyme with "eternally" is imperfect to modern ears, but the pairing of eternal waking with death's own death hardly needs a perfect chime. "One short sleep past" shrinks the whole of human mortality into a nap, so the verse itself makes death seem small.
The sonnet is significant partly because it makes an abstract fear feel personal and almost comic, and partly because its logic is not watertight. The comparison of death to sleep is an assertion more than a proof, and it works because of the confidence with which it is delivered. A reader might even sense, judging by how hard the speaker protests, a person talking himself into courage. That tension is the poem's strength. It shows faith as something argued, performed and fought for, with wit as the weapon, and it leaves readers with a line that is hard to forget because it describes death as something that can itself be killed.