I was brought up Protestant. One day a well-meaning neighbor asked my mother how I could possibly want to study the Middle Ages: "After all, they were all Catholic then."
This is a common belief but not strictly true. Protestantism, like modern Catholicism, is an heir to medieval Christianity. Modern denominations are the product of a good three thousand years of monotheism growing, enlarging, and then splitting. Ancient Judaism gave rise both to Christianity and to more recent versions of Judaism. Early Christianity split into Eastern Orthodoxy and western (Latin) Christianity. Islam borrowed from both Judaism and Christianity as well as from many revelations of its own. Medieval (Latin) Christianity split into Protestantism and Catholicism in the sixteenth century, and Protestantism has since split into numerous denominations.
Modern Catholicism differs from medieval Christianity in a great many ways, as does modern Protestantism. Yet Catholicism is happy to claim the Middle Ages as its own, stressing continuity, whereas Protestants sometimes act as though there were a curious thousand year gap between early Christianity and Martin Luther in the sixteenth century.
And of course, as I discuss in another post, it is counter-productive at best to label medieval Christianity (or for that matter any religion) as "superstition" just because it differs from one's own.
Medieval people took religion extremely seriously. Whereas modern America will spend millions on commercial buildings (think the Trump Tower), the biggest, grandest medieval buildings were the cathedrals, built for the glory of God. Modern Europeans, for the most part, still get married and buried and have their children baptized in churches built during the Middle Ages.
This did not mean that everyone was Christian. There were always small Jewish communities in western Europe, tolerated for much of the Middle Ages—though subject to intermittent persecution from about 1100 on. There were always plenty of doubters and scoffers, though all medieval accounts assure us that these people were appropriately struck down by the saints. For most people, most of the time, if you were baptized and said you were Christian, you counted as Christian. The church had far too much to do to inquire too closely into peasants' beliefs.
One will sometimes hear that "everyone had to obey the pope" in the Middle Ages, but this is far from true. For much of the Middle Ages, most people had no clue even who the pope was, much less what he thought. And when popes first asserted their authority as head of the church, they immediately got into knock-down, drag-out battles with the emperors.
But that is a story of another post.