This text charts and accounts for the remarkable growth of the Roman Catholic Church in Wales between the formation of its Province of Wales in 1916 and the commencement of the Second Vatican Council in 1962. This growth was at time when Noncomformity, hitherto predominant in Welsh religion, began its spectacular decline. It goes on to examine reaction to the expansion, both from within and outside the church. Catholics themselves became zealous in their enthusiasm as to the real possibility of the conversion of Wales to their faith, while many non-Catholics reacted with virulent hostility and prejudice. This reaction was heightened both as the growth came to be perceived as a political, and indeed Fascist, threat and as a number of prominent nationalists converted to Catholicisms. Anti-Catholic sentiment in Wales was strengthened by reactions to Papal dogma. Paradoxically, alongside this continuing animosity, there was also a gradual acceptance and reverence for the Roman Catholic Church, both for its individual members and for the Church at large.