The Enchiridion by St. Augustine
Chapter 29.--The Restored Part of Humanity Shall, in Accordance with the Promises of God, Succeed to the Place Which the Rebellious Angels Lost.
And so it pleased God, the Creator and Governor of the universe, that, since the whole body of the angels had not fallen into rebellion, the part of them which had fallen should remain in perdition eternally, and that the other part, which had in the rebellion remained steadfastly loyal, should rejoice in the sure and certain knowledge of their eternal happiness; but that, on the other hand, mankind, who constituted the remainder of the intelligent creation, having perished without exception under sin, both original and actual, and the consequent punishments, should be in part restored, and should fill up the gap which the rebellion and fall of the devils had left in the company of the angels. For this is the promise to the saints, that at the resurrection they shall be equal to the angels of God. And thus the Jerusalem which is above, which is the mother of us all, the city of God, shall not be spoiled of any of the number of her citizens, shall perhaps reign over even a more abundant population. We do not know the number either of the saints or of the devils; but we know that the children of the holy mother who was called barren on earth shall succeed to the place of the fallen angels, and shall dwell for ever in that peaceful abode from which they fell. But the number of the citizens, whether as it now is or as it shall be, is present to the thoughts of the great Creator, who calls those things which are not as though they were, and ordereth all things in measure, and number, and weight.