At first glance, the ‘binding’ in the title of this book refers to religio, understood as re-ligare, binding people back to a social unity. Augustine accepts this etymology, but he has much more to say. Always for him the bond of social unity is some form of love, situated somewhere along the spectrum from lust and greed to charity and the love of God – from concupiscentia to caritas – whether it be pirates drawn to the same pot of gold or Christians drawn by grace to union with God as the supreme Good. In twentieth-century scholarship, this emphasis on the unitive power of love regularly led to questions about the relation of Augustinian caritas to biblical agape and Platonist eros, all of which are ways of conceiving love for the divine. But it is long past time to investigate more closely the kind of love and social bond that was most important to many of the most eminent writers in the ancient world: friendship.

Phillip Brown situates that investigation at the intersection of three conceptual fields of force: classical notions of friendship; the controversy over the Donatist schism; and the hermeneutical resources that classical rhetoric afforded Augustine as a preacher addressing the controversy. The relation between the first two is the central thread of part i, with a chapter on the history of the Donatist schism, a chapter on the formation of Augustine's thinking about friendship and a chapter sketching his use of classical notions of friendship in his attempts to heal the schism.

Chapter i provides a history of ecclesial politics in Roman Africa, with the Donatist schism at its centre. Rooted in an African tradition of martyrdom going back to the great third-century bishop Cyprian, the Donatists formed a separate Church in the fourth century, rejecting the Catholic alliance with the empire after Constantine. When Augustine came on the scene as a Catholic bishop at the end of the century, he identified the underlying issue of the schism as the unity of the Church, a theme of Cyprian's that Augustine handled in a new way. The unity of the Church bound together as ‘one heart and one soul’ (Acts iv.32) is a form of friendship, the love that makes one soul out of two in classical thought – which Augustine expands to a vision of a societas in which love binds many souls so as to make them one, ex pluribus unum.

Chapter ii sketches the development of Augustine's experience of friendship, mainly as narrated in Confessions. Particularly valuable in this chapter are two quotations from Cicero that do not show up directly in Confessions but establish the classical concept of friendship shaping the narrative. Cicero traces that concept back to Pythagoras, for whom the intention of friendship is ‘to make one of many’ (‘ut unus fiat ex pluribus’) as ‘each loves the other as oneself’ (‘quisque altero delectetur ac se ipso’) in De officiis i.56. Moreover, Cicero's own definition of friendship requires agreement in belief as well as affection, in that friendship is nothing other than ‘agreement about all things divine and human, together with good will and charity’ (‘omnium divinarum humanarumque rerum cum benevolentia et caritate consensio’) in De amicitia 6.20, a text Augustine quotes in Contra academicos 3.13 and discusses in epistle cclviii. By this definition, a disagreement about religion is always also a breach of friendship.

Chapter iii narrows the focus to Augustine's attempt, in sermons preached soon after the imperial ‘Edict of Unity’ (405) imposed severe legal penalties upon the Donatists, to bring Donatists back into the unity of the Catholic Church, binding them back by the love of friends rather than mere fear of the law. True friendship, Augustine had said in his famous description of grief over the death of his best friend in Confessions 4.4.7, is possible only when souls are united by loving the one true God. Applied to the ecclesial situation, this meant that salvation depends on being members of the one ecclesial Body of Christ, bound back together by the love of God poured out by the Holy Spirit (Romans v.5). To be sure, until the end of the age the Body is not perfect: not just in Africa but in the whole world, the parable says, there are weeds sown among the wheat (Matthew xiii.38) – which means there are plenty of morally objectionable people present within the Catholic Church. But to separate oneself from them in order to form a pure Church, Augustine argues, is to refuse the friendship of Christ.

Part ii of the book offers a closer examination of Augustine's sermon series on the Gospel of John, especially sermons 1–16, preached early in 407 as the Catholic Church in Africa sought to be reunited with its Donatist neighbours under the imperial lash. What were the possibilities of friendship in such circumstances? Brown's quotations show us that Augustine's rhetoric in this situation is less overbearing and triumphalist than it might have been. Without compromising for a moment the claims of the Catholic Church to be in the right, he urges Donatists to join Catholics in friendship with Christ, and also urges Catholics to long for reunion with them without rancour and resentment.

Chapter iv considers the resources of classical rhetoric available to Augustine as he promotes this ecclesial friendship. Brown reviews rhetorical theory, such as the Ciceronian requirements of aequitas and decorum, and also highlights particular tropes that Augustine finds in the Scriptures. For example, the Church is the seamless garment of Christ, woven in unity from top to bottom (John xix.23), which is related to the wedding garment that is required if one is to be a friend of the bridegroom (Matt. xxii.12). And then there is the ancient trope of the Church as an ark of salvation like Noah's, to which Augustine adds the observation that the ark contained both crows and doves, for when a dove was sent forth, it returned, but the crow did not. ‘Who are the crows?’ he asks. ‘Those who seek what is their own. And who are the doves? Those who seek the things of Christ’ (In Joh. Evang. 6.2). This resonates with the image of the Church as God's ‘perfect one, the one dove’ (Song of Songs vi.9) which goes back to Cyprian. What is striking is that Augustine does not immediately say: ‘The doves are Catholics, the crows are Donatists.’ Evidently he does not want to make it seem too easy to separate the wheat and the weeds.

Chapter v identifies a key trope for ecclesial friendship in the figure of John the Baptist, who describes himself as the ‘friend of the bridegroom who stands and listens’ (John iii.29), which is to say the friend who loves and rejoices in Christ rather than himself. The passage must have had a particular resonance for Augustine's congregation, which would always be standing and listening during the sermon. It puts Augustine himself, and any other Catholic bishop, in the position of the friend of the bridegroom, who in humility says of Christ the bridegroom: ‘he must increase and I must decrease’ (John iii.30) and ‘he it is who baptizes’ (John i.33), in contrast to the proud claim of Donatist bishops to wield the sanctifying power of baptism themselves.

There is much that can be learned from this book, and I found that it brought me back to Augustine's sermons with eyes newly opened to key themes. The book is not so strong, however, on the intersection of Augustine's theology and philosophy, which really is an essential aspect of his thinking. It does not, for example, address how his insistence on ecclesial friendship is tied to Platonist eros or the love of wisdom. Also, there are some conceptual muddles, as when Brown describes the semiotic relation of spoken word and meaning (outward vox signifying an inner verbum) and illustrates it by quoting a passage relating spoken words and the eternal Word – evidently without noticing that this is an analogy, not an identity. Other muddles are minor but annoying, as when quotations include incomplete sentences and readers must guess at the missing subject or verb. There are also problems of English usage, such as when ‘Nevertheless’ begins many sentences that continue a previous thought rather than contrast with it. About halfway through the book one realises that the author does not quite know how to use this word. The overall effect of these muddles is a lack of readability and clarity in exposition, which is all too common in doctoral dissertations that should have been revised more thoroughly before publication. One wants to skim this kind of book for good ideas, rather than trust it as guide to one's own thinking.