Legend suggests the St. Gregory The good (d. 604) read the initial a few traces chanted by angels on a specific Easter morning in Rome when he walked barefoot in a fantastic spiritual procession and the saint thereupon included the fourth line: “Ora pro nobis Deum. Alleluia.” Almighty, everlasting God, https://paxtonqgseq.bloggazza.com/30434396/new-step-by-step-map-for-vita-pro-nobis