Some choice selections from Webster's Introduction:
Introduction: “The question has been proposed: Is the Church of Christ among the Catholics or among the Donatists? This needs to be determined from specific and clear citations in Holy Scripture. First, evidence is brought forth from the Old Testament and then from the New Testament.” (Augustine, Introduction, On the Unity of the Church. My emphasis)
. . .
"But, as I had begun to say, let us not listen to “you say this, I say that” but let us listen to “the Lord says this.” Certainly, there are the Lord’s books, on whose authority we both agree, to which we concede, and which we serve; there we seek the Church, there we argue our case" (Chapter 5). (My emphasis)
Webster says that Augustine basically says,
“Since both parties adhere to the truth of Scripture and believe them to be the word of God, it is scripture which should be the final arbiter.”
Augustine writes, “just as this doesn't need an interpreter” several times in his appeal to the Donatists. Augustine believed that theses Scriptures were clear and perspicuous, and did not need an infallible interpreter to settle the dispute.
In one of his sermons Augustine gives this exegesis of the rock of Matthew 16:
"Remember, in this man Peter, the rock. He’s the one, you see, who on being questioned by the Lord about who the disciples said he was, replied, ‘You are the Christ, the Son of the living God.’ On hearing this, Jesus said to him, ‘Blessed are you, Simon Bar Jona, because flesh and blood did not reveal it to you, but my Father who is in heaven. And I tell you’...‘You are Peter, Rocky, and on this rock I shall build my Church, and the gates of the underworld will not conquer her. To you shall I give the keys of the kingdom. Whatever you bind on earth shall also be bound in heaven; whatever you loose on earth shall also be loosed in heaven’ (Mt 16:15–19). In Peter, Rocky, we see our attention drawn to the rock. Now the apostle Paul says about the former people, ‘They drank from the spiritual rock that was following them; but the rock was Christ’ (1 Cor 10:4). So this disciple is called Rocky from the rock, like Christian from Christ. Why have I wanted to make this little introduction? In order to suggest to you that in Peter the Church is to be recognized. Christ, you see, built his Church not on a man but on Peter’s confession. What is Peter’s confession? ‘You are the Christ, the Son of the living God.’ There’s the rock for you, there’s the foundation, there’s where the Church has been built, which the gates of the underworld cannot conquer"
(John Rotelle, O.S.A., Ed., The Works of Saint Augustine (New Rochelle: New City Press, 1993), Sermons, Volume III/6, Sermon 229P.1, p. 327).
This treatise is of great interest historically because of what Augustine does not say constitutes unity. These words by Johann Joseph Ignaz von Dollinger, the most renowned Roman Catholic historian of the 19th century, who taught church history for 47 years, are very telling:
St. Augustine has written more on the Church, its unity and authority, than all the other Fathers put together. Yet, from all his numerous works, filling ten folios, only one sentence, in one letter, can be quoted, where he says that the principality of the Apostolic Chair has always been in Rome—which could, of course, be said then with equal truth of Antioch, Jerusalem, and Alexandria. Any reader of his Pastoral Letter to the separated Donatists on the Unity of the Church, must find it inexplicable...that in these seventy–five chapters there is not a single word on the necessity of communion with Rome as the centre of unity. He urges all sorts of arguments to show that the Donatists are bound to return to the Church, but of the Papal Chair, as one of them, he says not a word (Janus (Johann Joseph Ignaz von Dollinger), The Pope and the Council (Boston: Roberts, 1869), pp. 70-74).
Augustine says, 'Whoever dissents from Holy Scripture concerning the head is not in the Church' (Paragraph 7). (my emphasis)
He repeats this passage:
But, as I had begun to say, let us not listen to “you say this, I say that” but let us listen to “the Lord says this.” Certainly, there are the Lord’s books, on whose authority we both agree, to which we concede, and which we serve; there we seek the Church, there we argue our case (Augustine, On the Unity of the Church, Chapter 5).
"I do not wish the holy Church to be founded on human evidence, but on divine oracles" (Augustine, ibid., Chapter 6). (My emphasis)
"All such things then removed, let them demonstrate their Church, if they can, not in the speeches and murmurs of African, not in the councils of their bishops, not in the epistles of whatever debates, not in false signs and prodigies, since we are prepared and cautioned against them by the word of the Lord, but in the precept of the law, in the predictions of the prophets, in the songs of the psalms, in the utterances of the one shepherd himself, in the preaching of the evangelists, that is in all the canonical authority of the holy books, and not such that they might gather and cite things that are spoken obscurely or ambiguously or metaphorically which anyone might interpret according to his own opinion as he wishes. Such things cannot be properly understood and explained unless first those things that are said most openly are held with a strong faith (Chapter 47).