There is a Biblical psalm that says ‘Dust thou art, to dust thou returnest”. It means to say that we, human beings are made of dust and will return to dust again after our death. But the poet in his poem opposes this quote by the reasoning that this is applicable only of the body, the soul is indeed immortal.

Longfellow slams the pessimists who mournfully complain that nothing can be achieved in this life. According to the speaker, a person who spends all his time sleeping is already dead. Such worthless examples of life often misguide others. And he assures that life is not so shady or worthless as it looks like, and it has much more potential than we think of. To the speaker, this life is real.

As already said, the speaker (a young man) responds to the Biblical teachings that this human life is not important and that we are made of dust and eventually return to dust. So, we may take it as a psalm in response to a psalm, as the subtitle of the poem rightly suggests it.