The origins of the four migratory flows studied within the Euro-Mediterranean space differ in the way diverse factors weighed on the course of events. Significant though it be in the French and Spanish cases. Such representations as the unity of the “Christendom” and the unity of the “Muslim Ummah” undoubtedly suggest forms of belonging liable to mobilise whole populations but they do not operate at all times nor in all places. More importantly they compete with other forms of solidarity. We must therefore break with confessional historiographies that, in their treatment of events bestriding upwards of four centuries, overlook the dissentions between Christian monarchies faced with the threat of the Ottoman Empire, or the latter's will to subjugate predominantly Muslim societies that had, more or less energetically, fought its armies.
It remains that, right up to the dawn of our age, it is religion much more than nationality that held together such human communities as perceived themselves as a people. Then, those who spoke in the name of religion, which purported to connect the human soul to the revealed truth they professed, rejected all compromise: a truth given as unique is plainly not negotiable. A single common confessional axiom was thus the prevailing stance. This consideration radically sets apart Ancient Régime mindsets from the transformations realised over the past two centuries: nominally, Presidents of the French Republic may subscribe to any faith they choose, but they must be French nationals. Yet again, such observance is not universally valid.
After setting the problematic framework, having duly qualified the phrase “religious migrations”, remains the matter of estimating the size of the minorities under review. Sources require the deftest approach in this matter : an author who targets a minority may, as did Saint-Simon, boost its size to point up the potential danger or conversely, in the Ottoman case, under-represent the number of migrants to stress the strength of the majority. So, how many? 200,000 Jews, maybe twice as many Moslems, 150, 200,000 French Protestants, tens of thousands of Irish Catholics, up to 100,000 essentially Melkite of Maronite Shawam. Those figures do not tell us much; first because their absolute value must be set against the size of the home populations, next because the modes of departure varied considerably. However in each case the economic and cultural consequences were considerable.