Gregory Thaumaturgus,
Ante-Nicene Fathers Vol. VI, Dubious or Spurious Writings…
(1885)
“ The feast of All Saints is very ancient in the Oriental churches, and is assigned to the Octave of Pentecost, the Anglican Trinity Sunday. See Neale, Eastern Church, vol. ii. pp. 734, 753. In the West it was instituted when Boniface III. (who accepted from the Emperor Phocas the title of “Universal Bishop,” a.d. 607) turned the Pantheon into a church, and with a sort of practical epigram called it the church of “All the Saints.” It was a local festival until the ninth century, when the Emperor Louis the Pious introduced it into France and Germany. ”
