St Cyprian, third-century bishop of Carthage, developed a theory of church unity almost universally accepted up to the European Reformation: to be a member of the Body of Christ you needed to be in communion with a priest who was in communion with a bishop who in turn was in communion with all other bishops in the world. In his letters and treatises, St Cyprian wrestled with questions that arose in his defense on church unity, questions that continue to arise in various forms in the contemporary Church, and thus, these companion volumes are of ultimate value to the state of current Christendom.