In 1554 John Knox sought an interview with Heinrich Bullinger, Zwingli's successor at Zurich, in order to put some deeply troubling questions to him about the limits of political obligation. One of Knox's questions was ‘Whether obedience is to be rendered to a magistrate who enforces idolatry and condemns true religion’ (p. 223). Bullinger was clearly much alarmed by the implications of the enquiry, and answered that it was ‘very difficult to pronounce’ on such a topic, that he would need to have ‘an accurate knowledge of the circumstances’ before he could offer any advice at all, and that even then ‘it would be very foolish’ to try to say ‘anything specific upon the subject’ (p. 225). Bullinger's sense of panic is not surprising, but nor is Knox's sense of urgency. For Knox was asking the question against a background of growing fears about the whole future of the Protestant faith. After years of vacillation and compromise, the Catholic rulers of northern Europe had turned with violence against the reformers, and by the time of Knox's agonised enquiries they were engaged on a policy of reimposing religious unity by force.

The first country to experience this dramatic volte face was Germany. Abandoning any further attempts to negotiate with the princes of the Schmalkaldic League, Charles V moved his armies down the Rhine in 1543 and began to make plans for a holy war against the heretics.