Some keep the Sabbath going to Church — I keep it, staying at Home — With a Bobolink for a Chorister — And an Orchard, for a Dome — Some keep the Sabbath in Surplice — I just wear my Wings — And instead of tolling the Bell, for Church, Our little Sexton — sings. God preaches, a noted Clergyman — And the sermon is never long, So instead of getting to Heaven, at last — I’m going, all along. What is the central topic of the poem?