Well that is the thing- the society can be Christian without everyone in it having the Faith. Carl Schmitt once quipped of himself that he was "A Roman Catholic but not a Christian" and I have come across people in the TUV and DUP who would in a similar manner be "Protestants" without being Christians, again also you have people who are (neo-) Usuli Shia without really being Muslims. Charles Maurras who is one of the most important ideological architects of Christian Nationalism was himself an open atheist (there are claims that he converted to Catholicism on his death bed).
https://en.wikipedia.org/wiki/Charles_Maurras .
To me one of the worst arguments for ethno-nationalism is that people have to have homogeneous societies to avoid conflict- however conflict is just part of the human condition and will always find ways of arising (acknowledgement of this fact is one reason that I am not a Marxist). Radical/ethno-nationalist European movements in the post World War II West have been wracked by Religious conflicts between various types of Christians with each other but more so with mutual struggles between Christians, neo-Pagans and atheists. Politically if I was starting a Nationalist Party I would veer in the direction of keeping explicit Religion out of it. At the same time though I see real political threats to Christian civil and Religious liberties in the West today which concern me gravely- I notice also that "Bob" keeps away from discussing those threats.
I would argue for De Valera as opposed to French Jacobin style Liberalism or Catholic Political Integralism.