The United States of America has been called a "Christian nation." Yet what does that mean? More importantly, where did this idea come from? This work sets to answer both of those questions by looking at the development of "Protestant nationalism"--a movement following the Civil War that infused Protestant religious ideology with ideas of Americanism. At it''s core, Protestant nationalism was built on the idea of reform--reform for a country divided and reform of an American people who strayed from their faith. Those success however, transformed Protestant nationalism into a cultural movement, one that sought to define the nation in distinct socio-religious terms. From 1870-1920, this movement proved successful in that endeavor.