The Latin empire that western crusaders had set up at Constantinople in 1204 was bankrupt by the 1230s, and its emperor Baldwin II was pledging his most valuable assets to Venetian creditors. Among them was a relic venerated as the Crown of Thorns. Louis IX redeemed the pledge in 1239 and had the relic brought to France, walking the last stage barefoot in a plain tunic to receive it outside Sens. Further purchases followed, including a fragment venerated as the True Cross and other objects of the Passion, and the total outlay ran to a sum several times the cost of the chapel he then built to house them.
The Sainte-Chapelle, consecrated in 1248, is not a church of the usual kind. It sits inside the royal palace on the Ile de la Cite, is entered through the king's own precinct, and its upper chapel is walled almost entirely in glass, more than a thousand scenes of stained glass carried on stone mullions so slender that the structure looks impossible. The reliquary shrine stood raised at the east end. The building is an argument as much as a container: it places the relics of Christ's Passion in the keeping of the French crown, and by implication makes Paris a new Jerusalem and the king their guardian.
The acquisition also has a political reading that historians take seriously without treating the devotion as insincere. Louis was a genuinely ascetic man by every contemporary account, who wore a hair shirt, fed beggars at his own table and washed the feet of the poor. He was also assembling for the French monarchy a sacred standing that no other Western dynasty could match, and the Sainte-Chapelle, built by a king who was later canonized, is where those two things are hardest to separate.