“catholics being allowed into the United States in the mid 1800s” uhhh Catholics were in the 13 Colonies prior to the Revolution. Maryland was Primarily Catholic(the Maryland Calverts were Catholic(http://mdroots.thinkport.org/library/georgecalvert.asp).
Uh, Catholics were almost non-existent at the creation and founding of the United States, they were about 1.4% of the population, and when Catholic immigration started about the 1840s, by the 1850 Catholics became about 5% of America’s population.
America was almost 100% Protestant, just as Mexico used to be almost 100% Catholic.