But there is a narrower sense in which Christendom stands for a polity as well as a religion, for a nation as well as for a people. Christendom in this sense was an ideal which inspired and dignified many centuries of history and which has not yet altogether lost its power over the minds of men. Catholic Encyclopedia: “Christendom”
Christendom is the temporal and cultural sphere of Catholicity, as opposed to the spiritual(i.e. the Catholic Church), a uniquely Catholic Civilization with the Pope as its supreme head, and the Emperor is its temporal and military head. To separate Catholicism and Christendom is to severely maim Catholicism, and leave it without a political, cultural, and military defender. It is not a “supra-national state,” but a unifying Civilization encompassing many states and cultures in accordance with the Catholic principle of Subsidiarity, having the Holy Roman Imperium as its highest temporal authority.