The Piano Lesson
How we cite our quotes:
Doaker: "[…] so, on the Fourth of July, 191l…when Sutter was at the picnic what the county give every year…me and Wining Boy went on down there with him and took that piano out of Sutter's house." (1.2.119)
Notice, that they take the piano on July 4, Independence Day. When they take the piano, they are symbolically liberating their family from slavery. In a way, they are creating their own kind of Independence Day.
Stage Direction: Boy Willie begins to wrestle Sutter's Ghost. (2.5.201)
Boy Willie is wrestling the ghost of the man whose family once owned Boy Willie's family. In a way, you could see this wrestling match as an allegory for the African American race's struggle to overcome the historical aftershocks of slavery. Boy Willie is fighting to once again free his family from Sutter's oppression.
Avery: "Berniece, I can't do it." (2.2.202)
Christianity was taught to the slaves by their white masters. When the slaves first arrived from Africa they had their own spiritual beliefs and practices. However, the whites saw these as wrong. Given all this you could interpret Avery's failure to get rid of Sutter's ghost as the failure of Christianity itself. The "white" religion doesn't serve the African-American characters in their time of need.