In "Thanatopsis," what do lines 57 and 58 mean?

When we read this poem we are presented with a speaker that
celebrates and reflects upon Nature as a kind of mirror that matches his happy moods and offers
comfort and balm for his darker thoughts - especially those of death. In death, the speaker
argues, our individual beings intermingle with Nature's elements, and we join the company of all
who have gone before. The speaker advises us to live in such a way that when our time to die
comes, we can to to the grave sustained by trust, like a sleeper expecting pleasant
dreams.

Bearing this overall meaning of the poem in mind, let us
turn our attention to the context of lines 57 and 58. I will look at the lines around them too to
help us:

And millions
in those solitudes, since first

The flight of years began, have laid
them down

In their last sleep - the dead reign there
alone,

So shalt thou rest, and what if thou
withdraw

In silence from the living and no
friend

Take note of thy departure? All that
breathe

Will share thy
destiny.

In this part of the poem the
speaker is talking about what happens to us when we die. According to his view of the world, we
will join the "millions" who, "since first/The flight of years began" have died, or had their
"last sleep." Note how the author tries to make this prospect seem reassuring, creating a world
where we will not be alone and a fate that all who "breathe" will share.