stylistic:tone
Religion

The perception of Louis IX by his contemporaries as the exemplary Christian prince was reinforced by his religious zeal. Louis was an extremely devout Catholic, and he built the Sainte-Chapelle ("Holy Chapel"), located within the royal palace complex (now the Paris Hall of Justice), on the Île de la Cité in the centre of Paris. The Sainte Chapelle, a prime example of the Rayonnant style of Gothic architecture, was erected as a shrine for the Crown of Thorns and a fragment of the True Cross, precious relics of the Passion of Christ. He acquired these in 1239–41 from Emperor Baldwin II of the Latin Empire of Constantinople by agreeing to pay off Baldwin's debt to the Venetian merchant Niccolo Quirino, for which Baldwin had pledged the Crown of Thorns as collateral. Louis IX paid the exorbitant sum of 135,000 livres to clear the debt.

In 1230, the King forbade all forms of usury, defined at the time as any taking of interest and therefore covering most banking activities. Louis used these anti-usury laws to extract funds from Jewish and Lombard moneylenders, with the hopes that it would help pay for a future crusade. Louis also oversaw the Disputation of Paris in 1240, in which Paris's Jewish leaders were imprisoned and forced to admit to anti-Christian passages in the Talmud, the major source of Jewish commentaries on the Bible and religious law. As a result of the disputation, Pope Gregory IX declared that all copies of the Talmud should be seized and destroyed. In 1242, Louis ordered the burning of 12,000 Talmudim, along with other important Jewish books and scripture. The edict against the Talmud was eventually overturned by Gregory IX's successor, Innocent IV.

Louis also expanded the scope of the Inquisition in France. He set the punishment for blasphemy to mutilation of the tongue and lips. The area most affected by this expansion was southern France, where the Cathar sect had been strongest. The rate of confiscation of property from the Cathars and others reached its highest levels in the years before his first crusade and slowed upon his return to France in 1254.

In 1250, Louis headed a crusade to Egypt and was taken prisoner. During his captivity, he recited the Divine Office every day. After his release against ransom, he visited the Holy Land before returning to France. In these deeds, Louis IX tried to fulfill what he considered the duty of France as "the eldest daughter of the Church" (la fille aînée de l'Église), a tradition of protector of the Church going back to the Franks and Charlemagne, who had been crowned by Pope Leo III in Rome in 800. The kings of France were known in the Church by the title "most Christian king" (Rex Christianissimus).

Louis founded many hospitals and houses: the House of the Filles-Dieu for reformed prostitutes; the Quinze-Vingt for 300 blind men (1254), and hospitals at Pontoise, Vernon, and Compiègne.

St. Louis installed a house of the Trinitarian Order at Fontainebleau, his chateau and estate near Paris. He chose Trinitarians as his chaplains and was accompanied by them on his crusades. In his spiritual testament he wrote, "My dearest son, you should permit yourself to be tormented by every kind of martyrdom before you would allow yourself to commit a mortal sin."

Louis authored and sent the Enseignements, or teachings, to his son Philip III. The letter outlined how Philip should follow the example of Jesus Christ in order to be a moral leader. The letter is estimated to have been written in 1267, three years before Louis's death.