What is the most meaningful stanza in the poem "Psalm of Life" and why?

The poem's second stanza carries great meaning:

"Life is real! Life is earnest! 
   And the grave is not its goal; 
Dust thou art, to dust returnest, 
   Was not spoken of the soul."

Henry Wadsworth Longfellow's speaker suggests to the reader that simply living out our mortal lives is not fully living out our potential. He asserts that we begin and end as dust, but that our souls aren't concerned with the mechanics of our brief earthly lives. Overall, the poem's message is that great men know that they must take action to achieve immortality. The speaker contends that it is up to us to make our lives "sublime," and that in doing so, we are not only remembered, but also help others when they are feeling abandoned, lonely, and despairing. The poem's overall sentiment is that great men live lives that serve as inspiration to those who come along after them. This stanza speaks directly to the poem's larger theme.