Catholics are Christians within the Roman Catholic Church, the oldest denomination of Christianity, tracing its roots back to Jesus Christ and his Apostles in the first century CE. The Roman Catholic Church has a complex system of governance headed by the Papacy in the Vatican City, an independent nation within Rome, Italy, considered the world's longest continual absolute monarchy.

In the nineteenth century United States, the Catholic population expanded dramatically in tandem with increased immigration. In the 1840s, social upheaval in Catholic-majority nations like Ireland and Germany pushed many immigrants from these nations to the United States. The ties between immigration and Catholicism in the nineteenth-century U. S. influenced the anti-immigrant "nativism" movement and the creation of the anti-immigrant, anti-Catholic Know-Nothing movement in the 1850s.

Catholic religious affiliation also heavily influenced the religious, ethnic, and racial tensions that produced the New York Draft Riots of 1863, wherein predominantly white Irish and German-born Catholics, feeling overrepresented in the 1863 draft for the Union Army and wary of making the Civil War a fight to end slavery, rioted and set fire to many buildings in the city and committed acts of violence against African Americans. Therefore, Catholicism, like other Christian denominations in the United States, represented many complex layers of religion, race, class, ethnicity, and gender (Britannica; Howe, What Hath God Wrought, 822-827; McPherson, Battle Cry of Freedom, 493).

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