According to MacCulloch, (p 57)
On the eastern and southern rim of Europe, Islam remained a threat until the end of the seventeenth century. Even when the activities of the Ottoman fleet were curbed after the battle of Lepanto in 1571 (chapter 7, p. 331), north African corsairs systematically raided the Mediterranean coasts of Europe to acquire slave labour; in fact they ranged as far as Ireland and even Iceland, kidnapping men, women and children. Modern historians examining contemporary comment produce reliable estimates that Islamic raiders enslaved around a million western Christian Europeans between 1530 and 1640; this dwarfs the contemporary slave traffic in the other direction, and is about equivalent to the numbers of west Africans taken by Christian Europeans across the Atlantic at the same time. Two religious Orders, the Trinitarians and Mercedarians, specialized in ransoming Christian slaves, and over centuries honed diplomatic expertise and varied local knowledge to maximize the effectiveness of this specialized work. Large areas of Mediterranean coastline were abandoned for safer inland regions, or their people lived in perpetual dread of what might appear on the horizon; this may well explain, for instance, why Italians lost their medieval zest for adventurous trade overseas.4 The fear which this Islamic aggression engendered in Europe was an essential background to the Reformation, convincing many on both sides that God’s anger was poised to strike down the Christian world, and so making it all the more essential to please God by affirming the right form of Christian belief against other Christians.
Interesting, at least to me, that doctrinal differences are sometimes sharpened by external pressure, rather than being dissolved. Perhaps something similar is now happening in the Arab world; for all the talk about the Muslim invasion of Europe, there is today a Christian army in Baghdad, rather than a Muslim one outside Vienna.