This thesis looks at ways in which the British Catholic Church confronted the issue of Catholic unity and authority during the First World War. In a period when it was already attempting to articulate its position in relationship to the establishment and in the context of their Catholicity, the First World War offered the British Catholic Church both added difficulties and increased opportunity to express its position. For Catholics, the claim of universality was not only that they were the Church Universal in the sense that they were a supra-national church but that their Church was complete. Catholics argued that the Church was held together as a body united by and under the authority of Christ, the pontiff of Rome and the traditions maintained and accepted by the Church. These factors made it necessary for Catholics not only to make evident the advantage of their practices but to demonstrate that the fullness of the Church in its sacraments, doctrines and structure was neither in internal religious conflict nor fragmented by political or cultural differences; in short, that it was in itself complete. In the context of a world war in which Catholics were fighting one another and an unresolved political situation in Ireland, maintaining this position was both complicated and yet vital to the Catholic understanding of unity, authority and universality. In this thesis are analysed some of the ways in which the British Catholic Church addressed these challenges of self-definition.