Auguste Perret's Notre-Dame du Raincy is the turning point that lifts reinforced concrete from a cheap material imitating stone to an architectural material speaking in its own voice. It is called 'the cathedral of concrete' — rightly so, for it solves with reinforced concrete, using far less material, the problem the Gothic spent centuries trying to solve in stone.

The Gothic cathedral needed the complex balance of rib, buttress, and stained glass to free the wall from bearing and let light in. In Perret, steel-reinforced concrete does this alone: very slender, delicate concrete columns — only 30–40 cm across — carry the entire roof load. Because the columns carry the load, no bearing duty is left to the walls. So Perret removes the wall entirely and replaces it with a grille of factory-cast, geometrically perforated concrete blocks (claustra), with coloured glass set into the openings. The result is a near-immaterial shell that admits light from top to bottom — a modern, industrial stained glass.

Perret's radicalism lies in not hiding the material: the concrete is not rendered, painted, or made to look like stone; it is left bare, carrying the wood-grain texture of its formwork on its surface. What the structure is, is what is seen. Notre-Dame du Raincy proves that reinforced concrete can produce not only a strong but also an elegant and luminous architectural expression, opening the path of the concrete tradition that runs from Le Corbusier to contemporary church architecture.