Saint Mary's Parish was the town of Grantville's native Catholic church. Before the Ring of Fire, it was called Saint Vincent de Paul's, which created an awkward situation after Grantville landed in the past, as Vincent de Paul was still alive in 1631, and not all that highly regarded outside of France. Thus, it became St. Mary's.
The parish priest, Father Lawrence Mazzare, became famous after dispatches from the various spies sent into the alien town almost always reported on the Catholic parish, its priest, and how well they get along with the many Protestant sects and Jews.
Because of his reputation, Mazzare was selected to be Grantville's Ambassador to Venice. The United States of Europe desperately needed to open up trade for raw materials unobtainable elsewhere because of the war with the League of Ostend. When he went on this mission for the state, he hired none other than the famed Jesuit polymath Athanasius Kircher, who was attracted to the scientific and technical wonders of Grantville around 1632-1633.