Lines 7-8
And soonest our best men with thee do go,
Rest of their bones, and soul's delivery.
Billy Joel had it right, man: only the good die young.
The "best men […] soonest" follow this dude Death into the afterlife, thinking that he will give the "rest of their bones," and free or "deliver" their Christian souls from all the pain of earthly life.
(Note that "deliver" can also refer to childbirth, which adds to the whole "new life" idea.)
They are the hardest-working and bravest people in society, so they get to kick their feet back and enjoy eternal rest before everyone else.
(We think that, if Donne lived today, he would include women in this group, as well.)
The speaker almost certainly refers to people like soldiers and martyrs, who sacrifice themselves for the greater good.
Is Donne being too cute here?
After all, not that many soldiers are really thrilled to go off to war, and few people go to war intending to die – otherwise they wouldn’t be very good soldiers.
Donne makes it sound like the best men volunteer for death, when, in most cases, they only volunteer to risk death in order to achieve something else.
It is worth keeping in mind how downright sneaky this poem can be.
It almost makes you want to run out and take on one of the "World’s Most Dangerous Jobs.