Saint Augustine on Unity

"Small wonder that pride gives birth to division, and love to unity. But our catholic mother is herself a shepherd; she seeks the straying sheep everywhere, strengthens the weak, heals the sick, and binds up the injured. ... Thus she is like a vine that is spread out everywhere in its growth. The straying sheep are like useless branches which because of their sterility are deservedly cut off, not to destroy the vine but to prune it. When these branches were cut down, they were left lying there. But the vine grew and flourished, and it knew both the branches that remained upon it and those that had been cut off and left lying beside it. She calls the stray sheep back, however, because the Apostle said in reference to the broken branches: God has the power to graft them on again. Call them sheep straying from the flock or branches cut off from the vine, God is equally capable of calling back the sheep or of grafting the branches on again, for he is equally the chief shepherd and the true farmer. ... My sheep, he says, hear my voice and follow me.... [A]ll good shepherds are one in the one shepherd. It is not that good shepherds are lacking; they are there in the one shepherd. When we speak of "many" we refer to those who are divided from each other. Here only one is spoken of, because in this passage unity is commended. The reason why shepherds are not mentioned here, but only one shepherd, is not because the Lord has failed to find anyone to whom to entrust his sheep; he entrusted the sheep to Peter because he had found Peter. Indeed, in the case of Peter he also commended the unity of the flock. There were many apostles, and yet to one only did he say: Feed my sheep.... When he entrusted his sheep to Peter as one person to another, Christ chose to make Peter one with himself. He wanted to entrust him with the sheep in such a way that he himself might be the head and Peter might represent the body, that is, the Church. As bridegroom and bride, Christ and the Church were to be two in one flesh. Accordingly, what does he say before he entrusts the sheep to Peter as to someone who is not separate from himself? Peter, do you love me? He answered: I love you. And again: Do you love me? He answered: I love you. And a third time: Do you love me? He answered: I love you. He receives an assurance of love in order to establish unity. ... All should speak with one voice in Christ, not with different voices. Brethren, I beg all of you to say the same thing, and to have no dissensions among you. The sheep should hear this voice, a voice purified from all schism, freed from all heresy, and so follow their shepherd, who says: My sheep hear my voice and follow me."

- St. Augustine, Sermo 46.