The big dream and hope in the poem is to defeat Death and go to Heaven. The speaker is confident that his faith in God won’t let him down, but that’s the thing about faith: it doesn’t come with a guarantee. Unfortunately, the speaker doesn’t have a lot of other options. He must die, no matter what. Although he states the final assertion – "Death, thou shalt die" – as if it’s a fact of life (like gravity), it’s really an expression of hope for the unknown future.
The speaker’s fear of death betrays his uncertainty about whether his faith is enough to enter eternal bliss.