A lighthouse is a friend indeed for a lost ship to see from far away. But as illustrated in this picture, it is insufficient without some lower lights to show the boundaries of a harbor.
Jesus is the Light; he needs men and women who will point the way to a safe harbor. He needs the lower lights to guide the struggling seafarer safely to the shore.
The history of the song, “Let the Lower Lights be Burning,” is online at https://iblp.org/sites/default/files/pdf/responsibility_song.pdf