"Who can doubt whether these four verses are of mystical signification and that therefore this song is designed as a mystical song, and not a mere human love song? What should lead earthly lovers in singing of each other's beauty and love, to take notice of such things as these?
If we suppose the song to be mystical and that 'tis concerning Christ and His Church, a good account can be given of these passages, a very natural interpretation analogous to other Scriptures. By the "little sister" we may suppose is meant the same as the younger brother in the parable of the prodigal and by Solomon's vineyard, the same with Christ's vineyard so often spoken of in Scripture and by the "keepers" the same with the keeper of the vineyard and labourers in the harvest in Christ's parable who partake of the fruit of their labours with Christ".
Jonathan Edwards - "The Blank Bible" - Yale University Press, Illinois (2006) - (p627).