Though the Dutch were tolerant against foreigners (Portuguese Jews, for example, were warmly welcomed in Amsterdam), they expected of their countrymen to convert to Calvinism after the Reformation. Catholics were not allowed to conduct their worship in public. The catholic minority built clandestine churches (Dutch: schuilkerken): hidden churches that were not recognizable as such from the street.
One of those is “Our Lord in the Attic”, a secret church placed in the loft of a canal house at the Oudezijds Voorburgwal, in the current red light district, in Amsterdam. It was opened in 1661 and has been in use for catholic services for over 200 years, until the Basilica of St. Nicholas was opened. From 1888 on, the attic church has been a museum. However, since recently, the catholic mass is occasionally being celebrated again. Prayers next to moans - only in Amsterdam ;-)