That Milton was fiercely anti-Catholic is a commonplace, yet he never refers to those he opposed as 'Catholics' nor to their religion as 'Catholicism', and certainly not 'Roman Catholicism'. That his terms were instead 'Papist', Popery', and so on, is equally well recognised, but treated merely as an accident of contemporary polemical rhetoric. Their true significance is obscured by our practice of using 'Catholic' to stand for all Milton's references to the Church of Rome. This essay explores what appalled him in the 'principalitie of Rome' and the contrasting nature of the 'reformed catholic church' for which he argued, whose true catholicity, based on the liberty of the individual conscience and of independent congregations, welcomes diversity and debate and is doctrinally and ecclesiologically inclusive. However, Milton's stress on the 'industry of free reasoning' and the Christian duty to 'prove all things' leads to a different kind of exclusivity.