A working definition
Christian nationalism is a tight fusion of national identity and of allegiance to Christian religiosity so as to lead adherents to use political and cultural means to achieve compliance with and implementation of Christian practices and ethics — as perceived and defined by those adherents — in and by society as a whole.
America-specific definitions
Historian Randall Balmer offers a straightforward definition: a Christian Nationalist believes at least two of the following three statements: 1) America was founded as a Christian nation; 2) America is a Christian nation; or 3) America should be a Christian nation.
Sociologists Andrew L. Whitehead and Samuel L. Perry define it as “a cultural framework — a collection of myths, traditions, symbols, narratives, and value systems — that idealizes and advocates a fusion of Christianity with American civic life.” They note that the Christianity of Christian nationalism includes “assumptions of nativism, white supremacy, patriarchy, and heteronormativity, along with divine sanction for authoritarian control and militarism.” (Taking America Back for God, p. 10)
Scholar Katherine Stewart describes it as “not a religious creed but a political ideology. It promotes the myth that the American republic was founded as a Christian nation. It asserts that legitimate government rests not on the consent of the governed but on adherence to the doctrines of a specific religious, ethnic, and cultural heritage.” (The Power Worshippers, p. 4)