The sixteenth century was an age of prophets. Luther, Zwingli and Calvin interpreted the Word of God, challenging the claim of the Roman church to its own uniquely valid interpretation. These prophets found their armed champions in Knox, Coligny and William of Orange. The Catholic church countered them with her own arms, with Loyola, with Philip II and the Inquisition. But for more than a generation, before the religious conflicts erupted into open war, the political and religious life of Europe was dominated by a very different fighter for God: The Habsburg Emperor Charles V, the last medieval emperor to whom the religious and political unity of Christendom was both the ideal purpose of his life and a practicable object of policy. ‘Caesar is not a doctor of the gospels’, wrote Erasmus in his dedication to Charles of his paraphrase of St Matthew, ‘he is their champion.’ ‘God’s standard bearer’, the emperor called himself when, in June 1535, he weighed anchor at Barcelona to wrest Tunis from the Turks.

Charles had good reasons for his belief. ‘God has set you on the path towards a world monarchy,’ said the Grand Chancellor Gattinara, in 1519. Marriage alliances and inheritance had given Charles his unique opportunity. In the fifteenth century the houses of Austria and Burgundy had become united in northern Europe, those of Aragon and Castile in the south; but the marriage of Philip of Burgundy and Joanna of Castile, in 1496, produced a similar union between the northern and southern houses only through a series of unexpected deaths.