On Angelic Immutability

St. Gregory the Dialogist ca. 540-604

The Angels were created such that if they wished they could continue steadfast in the light of blessedness, but if they did not so wish they could also fall. Consequently even Satan fell with his legions of followers. But after his fall the Angels who remained steadfast were strengthened so that they certainly could not fall. This is well documented in the historical account at the beginning of the Book of Genesis, because God created Heaven which He then called the firmament. Therefore the Heavens were those which were first well made but then called the firmament because they received the virtue of immutability so that they certainly should not fall. (Homilies on the Book of Ezekiel, Fragment I: On Angels)