Legend suggests the St. Gregory the Great (d. 604) read the 1st 3 strains chanted by angels on a particular Easter morning in Rome when he walked barefoot in a terrific religious procession and which the saint thereupon additional the fourth line: “Ora Professional nobis Deum. Alleluia.” Almighty, eternal God, https://nervocure10752.thekatyblog.com/30226414/vita-pro-nobis-secrets