This article examines the relationship between religious debate and constitutional conflict in the 1590s, focusing on the status of ecclesiastical law and the right of the church courts to impose ex officio oaths upon English subjects. It argues that Richard Cosin, a client of Archbishop Whitgift and the leading apologist for the government's use of ex officio oaths, used the issue to make a series of aggressive and controversial assertions of state power. These theoretical claims did not involve sovereignty or the powers of the monarch – the issues usually addressed by historians of political thought – but rather the much more theologically charged question of the line between public authority and private conscience. As such, Cosin and his supporters transformed the raw materials of conformity and anti-puritanism into a view of the state and its coercive powers that seemed to threaten both the common law and the Elizabethan regime's own claims that it did not make ‘windows into men's souls’.