The Archdiocese of Mechelen-Brussels is unusual even among Catholic dioceses. It is the primatial see of Belgium, which means its archbishop holds a ceremonial first-among-equals status over every other Belgian bishop. And it has not one cathedral but two: Saint Rumbold's in Mechelen, the gothic giant whose unfinished tower has anchored the city's skyline since the 13th century, and the Cathedral of Saint Michael and Saint Gudula in Brussels, the brabantine gothic where Belgian royals are married and buried. The archdiocese was carved out in 1559 as part of a Catholic reorganization that came on the eve of religious war in the Low Countries. Its territory covers the former Province of Brabant, plus a handful of municipalities in Antwerp Province including Bonheiden, Duffel, Mechelen itself, and Sint-Katelijne-Waver.

Cardinal Mercier was appointed archbishop in 1906 and made cardinal the following year. When the Germans invaded Belgium in August 1914, sweeping through Mechelen toward the Marne, Mercier stayed. His Christmas pastoral letter of 1914, titled Patriotism and Endurance, was read in churches across occupied Belgium despite German attempts to confiscate it. He wrote that loyalty to occupiers was not required of citizens, that resistance was a Christian duty, that the wounded were owed care without question. The German governor placed him under house arrest in his palace at Mechelen, then released him under international pressure, then arrested him again. Mercier survived the war. He died in 1926. His Christmas letter is still quoted in Belgian schoolbooks. The Cathedral of Saint Rumbold, where he is buried, became a kind of national shrine in the years after.

Belgian federalism is famously complex, and the Catholic Church had to keep up. In 1995, the Belgian state split the old Province of Brabant into three: the Dutch-speaking Flemish Brabant, the French-speaking Walloon Brabant, and the bilingual Brussels-Capital Region. The Church declined to follow exactly. Rather than create three new dioceses, it kept Mechelen-Brussels intact and instead established three vicariates general within it, each headed by an auxiliary bishop, each aligned with one of the new civil regions. On the same day, the Diocese of Antwerp was carved out from territory the archdiocese had previously administered. Six years later, the Diocese of Hasselt followed. The result was a Belgian Catholic map that loosely mirrored the secular provinces while preserving the medieval primacy of Mechelen.

Cardinal Godfried Danneels led the archdiocese from 1979 to 2010, a thirty-one-year tenure that made him one of the most visible Catholic figures in Europe. He was named cardinal in 1983 and was considered papabile in two conclaves. His successors inherited a different church. In 2010, revelations of decades-long sexual abuse by Belgian clergy, including a bishop who had abused his own nephew, broke into open scandal. Investigators raided Danneels's residence; recordings emerged in which he had urged a victim to wait, to be discreet, to spare an aging abuser embarrassment. The damage to the archdiocese's moral authority was severe and lasting. Andre-Joseph Leonard took the chair in 2010 amid the crisis. Jozef De Kesel, named cardinal by Pope Francis in 2016, oversaw the slow work of rebuilding trust. Luc Terlinden, installed on 3 September 2023, is the current archbishop. The Belgian press has called his job, with some justice, an almost impossible mission.

The Catholic Church in Belgium has shrunk dramatically. Belgium was overwhelmingly Catholic into the 1960s; today only a minority of Belgians identify as practicing, and the cathedrals that Mercier once filled stand half-empty for ordinary Sunday Mass. The archdiocese has consolidated parishes, closed seminaries, and watched vocations dwindle. Yet the institutions endure. Saint Rumbold's still rings its 49-bell carillon over Mechelen. The royal weddings still happen at Saints Michael and Gudula. The line of archbishops since 1559, beginning with Cardinal Antoine Perrenot de Granvelle and stretching through fifteen Cardinals to Luc Terlinden, remains unbroken. A line of Belgian history that started before there was a Belgium continues, quieter now, in the same two cathedrals that began it.