Christianity means putting Christ first, Nationalism means putting ones nation first, how can one do both?
Your mistake is to assume that language is logical
The important thing here is the belief ("ism") in a Christian nation. The "ism" part really applies to the two words, not just to the word "nation".
Christianity is supposed to be about God, and loyalty to Him.
Statecraft is about getting a country to run.
The two are mutually exclusive, and deform each other two the point that anyone advocating either one,is unlikely to be impressed at the results.
Sure. But the Christian nationalists don't happen to agree.