Nat Turner, the subject of the poem, was an American slave who led a violent rebellion that lead to the deaths of sixty white people. Hayden wrote other poems about the brutal injustices of slavery and how some blacks reacted with violence - Middle Passage, for example, is about the slave uprising on the ship Amistad, portrayed in the film of the same name.
In that poem, the violence of the slaves and their leader, Cinquez, is not condemned but praised:
The deep immortal human wish,
the timeless will: I Cinquez
its deathless primaveral image,
life that transfigures many lives
Although later in his life Hayden would come to believe that violence was futile, in his earlier poems he defends it as necessary in defence of freedom. Although not related to slavery, see also A Ballad of Remembrance.
What has this to do with Nat Turner? Because the "harshening" he is asking of God is a steeling of his nerve to commit the acts of violence the poet sees as a necessary reaction to his condition.
In poem, Turner is wandering in the woods and experiences a vision heaven which he understands as a representation of a battle between slaves and masters. He is, therefore, to be understood as a civilized man, a Christian who has Christian values: including the instruction to "love thy neighor" and "thou shalt not kill".
Yet he knows these values were also used to justify slavery, and to encourage its victims to quietly accept their condition. Faced with the hyprocritical, un-Christian institution of slavery, he understands his condition is one of simple duality:
In scary night I wandered, praying,
Lord God my harshener,
speak to me now or let me die;
speak, Lord, to this mourner.
He must either steel himself to commit acts of brutality or "die" from his lack of freedom. He is a "mourner" not only because he mourns his enslaved condition, but because only through violence, through the breaking of Christian values, can he relieve it. He is asking God for the ability - and perhaps the forgiveness - to do so.
Similarly, when he addresses Jehovah as "my honer, harshener", he is asking to be honed into an instrument of just retribution. He is understanding, asking his God that the non-violent aspects of his faith, stressed by white preachers can instead be interprested as one of radical liberation.
- Robert Hayden, Collected Poems, Edited by Frederick Glaysher 1985