Keith Getty wrote the melody of this carol for his wife Kristyn when she was pregnant with their first daughter Eliza. Kristyn then wrote the lyrics as a simple carol for children to sing about how Christ took on humanity so that we could become children of God.
Watch here for an unplugged video of Keith & Kristyn Getty performing Jesus Joy of the Highest Heaven with their daughter Eliza: http://www.youtube.com/watch?v=v1ZTl1jES3o