The oldest and largest currently-active Christian denomination, the Catholic Church's roots go back to the period immediately following Jesus Christ's crucifixion. The church's central tenet is the belief in the holy trinity, the Father, the Son, and the Holy Ghost, and its belief that the pope is the human representative of God on Earth. The church's belief in the pope as well as several other practices led to the Protestant Reformation of the 16th Century, which resulted in the creation of various other denominations. During Abraham Lincoln's lifetime, the number of Catholics in the United States steadily grew due to increasing immigration from Germany and Ireland. This growing influx of Catholics partially spurred the nativist Know-Nothing movement which argued that Catholics should be prevented from holding public office because of their loyalty to the pope.

Jay P. Dolan, In Search of an American Catholicism: A History of Religion and Culture in Tension (New York: Oxford University Press, 2003); Theodore Maynard, The Story of American Catholicism (New York: Macmillan Company, 1960).