The Enchiridion by St. Augustine
Chapter 61.--The Church on Earth Has Been Redeemed from Sin by the Blood of a Mediator.
This part of the Church, then, which is made up of the holy angels and the hosts of God, shall become known to us in its true nature, when, at the end of the world, we shall be united with it in the common possession of everlasting happiness. But the other part, which, separated from it, wanders as a stranger on the earth, is better known to us, both because we belong to it, and because it is composed of men, and we too are men. This section of the Church has been redeemed from all sin by the blood of a Mediator who had no sin, and its song is: |If God be for us, who can be against us? He that spared not His own Son, but delivered Him up for us all.| Now it was not for the angels that Christ died. Yet what was done for the redemption of man through His death was in a sense done for the angels, because the enmity which sin had put between men and the holy angels is removed, and friendship is restored between them, and by the redemption of man the gaps which the great apostasy left in the angelic host are filled up.