St. Augustine of Hippo, The City of God (426)

“In the progress of the city of God through the ages, therefore, David first reigned in the earthly Jerusalem as a shadow of that which was to come.  Now David was a man skilled in songs, who dearly loved musical harmony, not with a vulgar delight, but with a believing disposition, and by it served his God, who is the true God, by the mystical representation of a great thing.  For the rational and well-ordered concord of diverse sounds in harmonious variety suggests the compact unity of the well-ordered city.”