The Gloria is modeled after the Bible's psalms and canticles, and was one of many early Christian hymns.
It is said that Pope Telesphorus (the second-century pope who helped set the date of Christmas) loved the Gloria so much that he ordered it sung at every Christmas Midnight Mass.
The Gloria gradually became part of morning prayer in both east and west. By the sixth century, it had become part of the Roman Mass - but only at Masses celebrated by a bishop on Sunday and feasts of martyrs. Priests could only use the Gloria at Mass at the Easter Vigil.
Throughout the years, the singing of the Gloria was gradually extended to choirs, and today it is usng by the people at Mass.
- Little Blue Book, Diocese of Saginaw