When Catholic missionaries first brought Christianity to the American Southwest, the Catholic population for many years was so sparse that it was administered from various dioceses in Old Mexico. By the time of the Mexican American War (1846-48), administrative realignments had become necessary for two reasons. First the Catholic population had grown large enough that creation of local dioceses had become advisable. Secondly with the Mexican American War, the acquisition by the United States of the area including the modern states of California, Nevada, Utah, Arizona, New Mexico and part of Colorado, creation of local dioceses had become necessary, so that American parishes and missions would not be administered across national lines.
By 1853, the Holy See had divided the Mexican Cession, as it was called, into two regions. The newly created Archdiocese of San Francisco, extended from the 42nd parallel (the present northern border of California, Nevada, and Utah) south to the 37th parallel (slightly north of Monterey, California). South of the 37th parallel were the Dioceses of Monterey and Santa Fe. Utah Territory, created as part of the Compromise of 1850, was located within the Archdiocese of San Francisco.
In 1866, Utah Catholics were briefly placed under the jurisdiction of the newly created Diocese of Marysville, California (presently the Diocese of Sacramento). Two years later, Utah was moved into the Diocese of Denver, but in 1870 it was returned to the Archdiocese of San Francisco, where it remained a suffragan diocese until 2023.
As the Catholic population within the former Mexican Cession continued to grow, eventually nine suffragan dioceses were erected within the Province of San Francisco. Utah was one of those. On November 23, 1886, the Vicariate Apostolic of Utah was established, and on January 27, 1891 it became the Diocese of Salt Lake under its first bishop, the pioneer priest Lawrence Scanlan. A papal decree of 1951 added the word “City” to the name of the diocese, making it the Diocese of Salt Lake City.
Until 1931, when the Diocese of Reno was created, six eastern counties of Nevada were part of the diocese, making the Diocese of Salt Lake geographically the largest diocese in the United States. By comparison, the seven dioceses in the state of New York comprise a total area of 47,246 square miles, which was only about one-third the area of the Diocese of Salt Lake. Even in its reduced size after 1931, Utah is still one of the larger dioceses in the country with it’s nearly 85,000 square miles.