City of God
Two works that changed Christianity forever. They shaped us and continue to shape us. We need to let them keep shaping us by keeping them in mind.
I think that's why the Pope keeps directing us to the patristics.
We believe we are the body of Christ, we have been the body of Christ and we will continue to be the body of Christ because Jesus Christ said so. We have to remember those who have been in order to continue to be.