Death is definitely the big idea in "Thanatopsis." From the title to the last line, this is a poem all about dying. It looks at the issue from a bunch of different angles. It deals with death across all time and all over the world. It talks about how it makes us feel, about how it is related to spirituality, and history and nature. Mostly it's purpose is to remind you that you are going to die, but death isn't something to be afraid of.
The speaker’s steady focus on death makes the idea seem familiar; the repetition of the central theme takes away some of the fear the reader feels.
"Thanatopsis" gives concrete examples of the terror and the inevitability of death, without giving the reader any clear reason to feel better about it.