Saint Louis was laid out in 1764 by French fur traders and named for the crusading king Louis IX, and it was Catholic before it was American. A log church went up within a few years, and the parish became a diocese in 1826 as settlement pushed west. The Old Cathedral of 1834 still stands beside the river, the oldest cathedral west of the Mississippi and now dwarfed by the arch built around it.
Its bishops used the city as a base for missions. Jesuits arrived in 1823 and worked among the nations of the plains, and the best known of them, Pierre-Jean De Smet, travelled tens of thousands of miles, established missions in the northern Rockies, and was repeatedly used by the federal government as an intermediary in treaty negotiations, including a journey to Sitting Bull's camp in 1868. He is remembered by some as an advocate and by others as an instrument of a policy that dispossessed the people he lived among, and both readings draw on the same record. Rose Philippine Duchesne came from France in 1818 and founded the first free school west of the river.
Saint Louis University, founded in 1818, was the first university west of the Mississippi. The newer Cathedral Basilica, begun in 1907 and finished eight decades later, is lined with mosaic covering some eight thousand square metres, one of the largest such installations anywhere.