Although the unionist literature of the post-1603 period has recently attracted a good deal of scholarly analysis, much less attention has been paid to the development of ideologies of British unionism in the century preceding James VI's accession to the throne of England. Historians are obviously aware that the propagandists of the early seventeenth century did not develop their ideas ex nihilo, but only Arthur Williamson has explored the ‘prehistory’ of British unionism in any detail. In particular, he has rightly highlighted the significance of the ‘Edwardian Moment’ of the late 1540s as a prime source of the triumphalist imperial rhetoric which came into its own with the union of the Anglo-Scottish crowns in 1603. Likewise, though rather less convincingly, he has argued that many of the reformers who presided over the Protestant revolution in Scotland in the years after 1559 were imbued with a similar vision of Britain's imperial destiny. Yet despite Williamson's pioneering research, the significance of these episodes in the history of the debate over Anglo-Scottish union remains largely unexplored. The main aim of what follows, therefore, is to re-examine the relationship between the Scottish Reformation and the development of British imperialism in the crucial decades of the 1540s and 1560s. Before doing so, however, it is important to review what meaning (or meanings) the idea of Britain possessed for contemporary Scots and Englishmen; for the unionists of the mid-sixteenth century, like those of 1603 and after, drew on a considerable heritage of ideas and assumptions about Britain's history and destiny whose roots lie deep in the middle ages.