Update: I’ve made better maps of these dioceses in this post.
It’s well known among American religious historians that the first Catholic diocese in the United States was the diocese of Baltimore, erected in 1789 under Bishop John Carroll. What I didn’t know was how Catholic dioceses spread across the United States, and how U.S. dioceses related to Canadian and Mexican dioceses. So I gathered data on Catholic dioceses in the United States, Canada, and Mexico, and mapped their spread across North America over time. Below are maps of 356 North American dioceses by decade, with brief comments on the nineteenth-century maps about what they reveal about U.S. Catholicism.
A few general comments: A map of Catholic dioceses is not, of course, a map of Catholics. The institution lags behind people. Nevertheless, I think that plotting dioceses is useful for understanding the institutional structure of the North American Catholic Church. In particular, as I’ll show in my comments below, combining Canadian and Mexican dioceses with U.S. dioceses provides a much fuller picture of North American Catholicism over several centuries.
In these maps, dioceses that are not within the bounds of the map have not been plotted, in particular some dioceses in Alaska, Puerto Rico, the Virgin Islands, Newfoundland and Labrador, New Brunswick, Nova Scotia, Prince Edward Island, and the very north of Canada. Red dots are dioceses; purple dots are archdioceses. I have included both Latin and non-Latin rite dioceses in the charts: you can look in the data to find the locations of non-Latin rite jurisdictions. I have included only dioceses, but not regions or institutions with ordinary jurisdiction but not episcopal character.
The map for 1790 shows the first U.S. diocese in Baltimore. But the map also shows the Catholic diocese in Quebec: the source of so much consternation to Protestants during the American Revolution. It also shows ten dioceses, including a metropolitan see, in Mexico, some of which had been established since the sixteenth century. There were also dioceses in the Caribbean not shown. The territory that became the United States was very late to establish this institution of the Catholic church.
By 1800 a diocese has been added in New Orleans, but it was under the jurisdiction of the French and not the United States, though it would become part of the United States through the Louisiana Purchase.
In April of 1808, Baltimore was elevated to be an archdiocese, with three new sees: Boston, New York, and Bardstown, Kentucky. While Baltimore in the mid-Atlantic state of Maryland remained the institutional center, and while Boston and New York were gaining new prominence, the western frontier was also important for of U.S. Catholicism.
By 1830 the U.S. dioceses continue to expand in the west and along the Atlantic seaboard. But a Canadian diocese in Kingston (before a diocese in Montreal) shows the importance of the Great Lakes and St. Lawrence River.
By 1840 there are more dioceses in the Mississippi River valley. But there are also two Mexican dioceses in California in territory which would become part of the United States in the next decade due to the Mexican-American War.
By the 1850s there were a number of new dioceses in the Great Lakes region in both the United States and Canada. New York, but also New Orleans, St. Louis, Cincinnati, and Portland, had been elevated to archdioceses. There is also a cluster of dioceses in the Pacific Northwest in the territory disputed by the U.S. and Canada.
By the 1860s most of the growth in dioceses has been on both sides of the Great Lakes.