After the Dutch Revolt swept the southern Netherlands free of Spanish rule, the new Dutch Republic took the Reformed Church as its public faith. Catholics, Lutherans, Mennonites, and Remonstrants found themselves in an awkward middle position. The Republic would not jail them for their beliefs, would not exile them, would not - except in flares of religious panic - actively persecute them. But it would not let them worship in public either. The historian Benjamin J. Kaplan called what emerged a 'semi-clandestine' arrangement. The 1648 Peace of Westphalia, the treaty that ended the Thirty Years' War, formalized the distinction in international law: there was public worship, there was private domestic devotion, and there was a middle category - 'exercitium religionis privatum,' private religious services held by clergy in houses or buildings designated for the purpose but not in churches at set hours. Hidden churches lived in that third box.

The rules were simple and absolute. No bells. No towers. No steeples. No crosses on the exterior. No icons. No architectural flourishes that announced what was inside. From the street, a schuilkerk had to look like its neighbors. Inside, it could be magnificent. Ons' Lieve Heer op Solder, built into the attic of a canal house, had pews for a hundred and fifty worshipers, a working pipe organ, a marble altar, and a ceiling painted to suggest a much grander sacred space. Amsterdam's Vrijburg, built in 1629 for the Remonstrants, took a different approach: a freestanding building hidden at the center of a city block, surrounded by houses on all four sides so that no public street could see it. In the countryside the disguise often went the other way - hidden Catholic churches there were built to look like barns, and were called schuurkerken. The most famous surviving example, St. Ninian's at Tynet in Scotland, looks like a long low cattle shed; only a small stone finial on the west gable hints at what happens inside.

Hidden churches needed art. They commissioned it carefully - the work could be magnificent inside, as long as it never escaped to the facade. Catholic congregations in the Dutch Republic supported some of the most important Dutch painters of the seventeenth century. Gerard van Honthorst, who had trained in Rome and brought Caravaggesque chiaroscuro back to Utrecht, painted altarpieces that hung over altars no street ever saw. Abraham Bloemaert, the great Utrecht teacher whose pupils included Honthorst and Cornelis van Poelenburgh, produced devotional paintings for hidden churches throughout the city. Pieter de Grebber in Haarlem, Jan Miense Molenaer, Claes Corneliszoon Moeyaert, Jan de Bray - half the major Dutch painters of the era worked for clients whose buildings were not supposed to exist. Some of those altarpieces still hang in the churches they were made for. Others were sold during nineteenth-century renovations and now turn up in museums labeled simply as 'religious scenes.'

The schuilkerken were a workaround, and like every workaround they outlived the problem they solved. After the French Revolutionary invasion of 1795 the Batavian Republic granted Catholics, Mennonites, and Lutherans full religious equality. The hidden churches were free to put up bells, build towers, paint crosses on the front. Many were torn down and replaced with the proud public Catholic churches of the nineteenth century - the neo-Gothic buildings that now dominate every Dutch city skyline alongside the Protestant ones. But some of the hidden churches survived intact. Ons' Lieve Heer op Solder became a museum in 1888 and is still open to the public, complete with its working organ and its painted ceiling. A Jewish house synagogue in Traenheim in Alsace - established in 1723 over the loud objections of the local Catholic pastor but with permission from the government - still has Hebrew prayers on the walls. They remain, all of them, evidence of a particular Dutch genius: the ability to let people be different without quite admitting that you have done so.