Legend suggests the St. Gregory the Great (d. 604) listened to the very first a few lines chanted by angels on a specific Easter morning in Rome when he walked barefoot in a great spiritual procession and which the saint thereupon additional the fourth line: “Ora Professional nobis Deum. Alleluia.” https://vita-pro-nobis54208.blogdosaga.com/31101375/facts-about-vita-pro-nobis-revealed