In the middle of the 20th century, a prolonged animosity came to an end. For more than four centuries, the enmity between Catholics and Protestants, known to theologians as the two confessions, had been one of the organising principles of European life. But, then, it stopped.

To grasp just how revolutionary this inter-Christian peace was, it’s worth remembering what came before it. Because the mutual hatred between the confessions shaped not only the early modern era, when gruesome acts of violence like St Bartholomew’s Day (1572) and the Thirty Years’ War (1618-48) tore Europe apart. Anti-Catholicism and anti-Protestantism remained powerful forces well into the late 19th and early 20th centuries, and shaped social and political life. The most extreme case was Germany, where the Protestant majority in 1871 unleashed an aggressive campaign of persecution against the Catholic minority. For seven years, state authorities expelled Catholic orders, took over Catholic educational institutions, and censored Catholic publications.

In the Netherlands, Protestant crowds violently attacked Catholic processions; in Austria, a popular movement called ‘Away from Rome’ began a (failed) campaign in 1897 to eradicate Catholicism through mass conversion. Catholics, for their part, were just as hostile to Protestants. In France, Catholic magazines and sermons blamed Protestants for treason, some even called for stripping them of citizenship. Business associations, labour unions and even marching bands were often divided across confessional lines.

Even on an everyday level, it still was common into the 20th century for neighbourhoods, parties and magazines to be strictly Catholic or Protestant. Prominent politicians and lay writers routinely blamed the other confession for backwardness, subversion and sexual perversity. A prominent German historian even claimed, in the 1860s, that Catholics and Protestants were descendants of different races.

But then, by the 1950s, this mutual disdain ended. The two confessions reconciled, lay leaders established joint organisations, and politicians even founded powerful interconfessional political parties. Even Church authorities, who for a while dragged their feet, ultimately came around. The Catholic Church, during the Second Vatican Council, officially declared in 1964 that Protestants were not heretics, but brethren in faith. Only in Northern Ireland did anti-Catholicism and anti-Protestantism remain powerful, a remarkable exception that proved the rule.

How did this shocking change come about? After four centuries of division, why did old animosities die so quickly? It’s easy to presume that this dramatic shift happened after the Second World War and was part of Europe’s broader liberalisation. After the trauma of Nazism and Stalinism, we may think, many Europeans came to appreciate pluralism. Or one might imagine that Catholic-Protestant peace came from the onward march of secularisation. People left the Churches in the 1960s, so they also cared less about old tensions.

But both these assumptions would be wrong. Because the Catholic-Protestant truce in fact began long before the Second World War, in response to the Nazis’ call to end religious discord and to instead forge racial unity. Many Catholic and Protestant thinkers and leaders were deeply impressed by this revolutionary message. Even if they disliked some of Hitler’s ideas, they believed that inter-Christian cooperation opened exciting new possibilities. More than anything, they hoped that unity would allow them to build a European order that was based on inequality. Under the Nazis’ hegemony, Catholic and Protestant leaders hoped to protect the economic hierarchy between workers and employers, and the sexual disparity between men and women. In its origins, that is, the peace between Catholics and Protestants entailed not just new tolerance, but also protection of harsh exclusion. And after the Second World War, this fact turned out to be hugely consequential, when Catholics and Protestants came to power and helped build a deeply unequal Europe.