Deseret includes, more or less, OTL Utah and western Colorado.
For some time, Mexico granted the residence near Gran Lago Salado, to some US citizens who escaped religious persecution back home.
In 1875, as the Mexican Civil War was over and Mexico had lost California, Texas, Yucatán, Campeche and Chiapas; Mexico and the USA granted independence to those settlements round Gran Lago Salado, or Salt Lake, as most Anglo settlers called it.
In 1963, a series of secular reforms claimed to shape Deseret as a modern state, granting religion freedom and recognizing the diversity of cultures that inhabited the territory. Still, the most practiced religion is Mormonism, and the Church of JC of LDS has a status as state religion.