The theme of mortality appears right from the first line, with the metaphor of the church bell ringing a "knell" for the end of the day. Since a "knell" is a bell toll for a person who has died, the speaker is personifying the day, and is also making death into a kind of universal—it's not just people who die, but even each day dies at sunset! Fortunately, the sun comes up again in the morning, so maybe there's a hint of hope here?
Each in his narrow cell for ever laid, The rude forefathers of the hamlet sleep. (15-16)
The speaker is using a euphemism to describe death here—he says that the dead villagers are just "sleeping." That also sounds hopeful, since after all, if you fall asleep, you're going to wake up again, eventually. Hey, maybe this death thing isn't so bad after all!
The cock's shrill clarion, or the echoing horn, No more shall rouse them from their lowly bed. (19-20)
Oh no, wait! These guys aren't just sleeping, after all—they're never going to get woken up by the rooster crowing again. So much for that hopeful idea. This poem just took a turn for the depressing.
Oft did the harvest to their sickle yield, (25)
Sure, this line might sound like it's just about farmers harvesting their grain, but did you ever stop and think about what the Grim Reaper (a.k.a. the personification of Death in some Western cultures) is up to? Check out this image. That scythe that the Grim Reaper is holding is also a farm implement, intended for—you guessed it—harvesting the souls of dead people. So it's possible that the sickle these farmers are using is intended to make us think of the grim reaper harvesting souls. Hmm. Sounds a bit morbid, doesn't it?
The paths of glory lead but to the grave. (36)
The speaker wants us to remember that everyone dies. EVERYONE. And all of your worldly ambitions—college, a great career, whatever your "paths of glory" might be—only end in one place: death. Sorry to rain on your parade!
Can storied urn or animated bust Back to its mansion call the fleeting breath? (41-42)
Here the speaker warns against putting too much emphasis on monuments and fancy mausoleums to commemorate yourself or a loved one after death. After all, no fancy-schmancy statue or "urn" is going to bring you back from the dead.