In this defiant poem, Donne's speaker takes on death with an aggressive swagger. People fear death and did so even more in the seventeenth century, when outbreaks of disease or almost any illness or accident could take a life suddenly.
Donne's speaker, rather than cowering, personifies "Death" as a person who can be jeered at because he has already lost the war against humankind. He has no reason to think that he is powerful, because he is a weak loser. The speaker tells Death not to keep deluding himself that he is
Mighty and dreadful, for thou art not so
The speaker states, too, things that Death thinks it can "overthrow":
Die not, poor Death, nor yet canst thou kill me
Contemporary readers, steeped in Christianity, would have quickly understood the context: because Jesus Christ died and was brought back to life by God, so all Christians will be resurrected at an appointed time.
The speaker goes on to say that Death is weak and dependent on human agency. Without "kings, and desperate men," Death has no chance to kill. Likewise, it is passively dependent on attaching itself to the emergence of unpleasant events like war or illness to function.
Death, too, is no more than a sleep from which we will awake. At the end of the poem, the speaker addresses Death directly with words of triumph. Humans will:
And death shall be no more
It is unusual for a religious poem to take on such an aggressive tone. The speaker sounds like a fighter emerging from a bar after a few drinks to challenge and beat up a hated enemy. By taking such a tone, Donne jolts the faithful reader into a new awareness of death's defeat.