Leo the Great

If the true high priest does not atone for us, using the nature proper to us, and the true blood of the spotless Lamb does not cleanse us, then a true priesthood and true sacrifices do not exist in any other way in God’s church, which is the body of Christ. Although he is seated at the right hand of the Father, he performs the sacrament of the atonement in the same flesh which he assumed from the Virgin Mary.

The bond of our unity cannot be firm unless the bond of charity has tied us together in indivisible solidarity.… It is the connection of the entire body which makes for one health, one beauty. And this connection requires the unanimity of the entire body, but especially it demands harmony among the bishops.