Legend claims 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 whilst he walked barefoot in a great spiritual procession and which the saint thereupon added the fourth line: “Ora Professional nobis Deum. Alleluia.” https://vitapronobis44208.blogs-service.com/62191383/5-simple-statements-about-vita-pro-nobis-explained