For Augustine, the City of God cannot be identified with “the Church” for the simple reason that the City of God contains angels as part of its membership (likewise, the City of Man contains demons). One place in which he discusses this is City of God 12.9 in his treatment of angels. Part of the City of God, then, is made up of mortal men, and part of that part is composed of those who are currently “sojourning on earth.” But men–whether living, dead, or yet to be born–do not account for the entire society.
Besides, this too has to be inquired into, whether, if the good angels made their own will good, they did so with or without will? If without, then it was not their doing. If with, was the will good or bad? If bad, how could a bad will give birth to a good one? If good, then already they had a good will. And who made this will, which already they had, but He who created them with a good will, or with that chaste love by which they cleaved to Him, in one and the same act creating their nature, and endowing it with grace? And thus we are driven to believe that the holy angels never existed without a good will or the love of God. But the angels who, though created good, are yet evil now, became so by their own will. And this will was not made evil by their good nature, unless by its voluntary defection from good; for good is not the cause of evil, but a defection from good is. These angels, therefore, either received less of the grace of the divine love than those who persevered in the same; or if both were created equally good, then, while the one fell by their evil will, the others were more abundantly assisted, and attained to that pitch of blessedness at which they became certain they should never fall from it—as we have already shown in the preceding book. We must therefore acknowledge, with the praise due to the Creator, that not only of holy men, but also of the holy angels, it can be said thatthe love of God is shed abroad in their hearts by the Holy Ghost, which is given unto them.Romans 5:5 And that not only of men, but primarily and principally of angels it is true, as it is written,It is good to draw near to God.And those who have this good in common, have, both with Him to whom they draw near, and with one another, a holy fellowship, and form one city of God— His living sacrifice, and His living temple. And I see that, as I have now spoken of the rise of this city among the angels, it is time to speak of the origin of that part of it which is hereafter to be united to the immortal angels, and which at present is being gathered from among mortal men, and is either sojourning on earth, or, in the persons of those who have passed through death, is resting in the secret receptacles and abodes of disembodied spirits. For from one man, whom God created as the first, the whole human race descended, according to the faith of Holy Scripture, which deservedly is of wonderful authority among all nations throughout the world; since, among its other true statements, it predicted, by its divine foresight, that all nations would give credit to it.