While I am not familiar with the song, I would tend to think of the song as a celebration of the incarnation (i.e. birth) of Jesus because of the imagery of the keys to the city of Bethlehem--Bethlehem being the city of David--and traditional birthplace of Jesus. The theme of the song is the fulfillment of the messianic hope and expectation in the fulfillment of several passages of the Old Testament. Isaiah, chapters 7,9 and 11 come to mind. The carrying of the palms of victory in her (Mary's) hand is especially reminiscent of the 11th chapter of Isaiah in which there is promised that a shoot will spring forth (from the otherwise rather dead) stump of Jesse--which is more familiarily celebrated in the Christmas Carol, "Lo, How a Rose e'er Blooming."

Now, I am not really an expert on the themes of these sorts of spirituals, but as the image from scripture is one of hope and the fulfillment of the promise of liberation, might it have been that the song was a sort of a code celebrating the hope of liberation from slavery?

Hope these comments are helpful, it is an interesting thread. Thanks for bringing the song to our attention!

Greg