In the aftermath of the First World War, the parish priest of Raincy decided to build a new church. But the country was ruined and the clergy's coffers were empty. One man rose to the double challenge of urgency and budget: architect Auguste Perret. At the time, building quickly and cheaply meant using reinforced concrete. An unloved and shameful material, reserved for industrial works. It took only 13 months to build this "Sainte chapelle du béton", a magnificent demonstration of the plastic and constructive qualities of this material, and a great lesson from a once-forgotten architect who revolutionized architectural language.