Prior to the fire, Notre Dame housed some of Christianity's most treasured relics - including a fragment of the Holy Cross and the entire Crown of Thorns.
In the 13th century, French King Louis IX, the future Saint Louis, set about buying as many relics as possible.
He spent approximately 50% of the French national budget to do so - acquiring the Holy Lance, the Holy Sponge and the Crown of Thorns in the process.
He also constructed the Sainte-Chapelle to house them - which can still be visited today.
During the French Revolution, the Sainte-Chapelle was looted, and its relics were moved to Notre-Dame for safekeeping.
These relics were added to Notre-Dame’s own collection - which included a white tunic belonging to Saint Louis, as well as fragments of his jaw, rib, and teeth, and bones and hair from Saint Denis and Saint Geneviève.
Before the fire, once a month the relics were publicly displayed, under the watchful eye of the Knights of the Holy Sepulchre, a special military order operating under the authority of the pope.
Not all relics stayed inside the cathedral, though.
Fun fact: In 1935, an archbishop placed the relics of Saint Denis, Saint Geneviève, and a thorn from the Crown of Thorns inside a weathervane at the top of the cathedral's spire to shield the congregation from lightning.
These relics were saved from the fire and are now housed at the Louvre.
