Nicene and Post-Nicene Fathers, Vol. III:
Doctrinal Treatises of St. Augustin: Chapter 28

Chapter 28.—When the Rebellious Angels Were Cast Out, the Rest Remained in the Enjoyment of Eternal Happiness with God.

Whilst some of the angels, then, in their p. 247 pride and impiety rebelled against God, and were cast down from their heavenly abode into the lowest darkness, the remaining number dwelt with God in eternal and unchanging purity and happiness. For all were not sprung from one angel who had fallen and been condemned, so that they were not all, like men, involved by one original sin in the bonds of an inherited guilt, and so made subject to the penalty which one had incurred; but when he, who afterwards became the devil, was with his associates in crime exalted in pride, and by that very exaltation was with them cast down, the rest remained steadfast in piety and obedience to their Lord, and obtained, what before they had not enjoyed, a sure and certain knowledge of their eternal safety, and freedom from the possibility of falling.