Technically this
>>17929891
But what we usually understand as nationalism typically ends up just being a form of secular paganism. Particularism, the worship of a nation-state or a cultural group, ethnocentrism, all of these clash with Christian core teachings.
Nationalism as in the promotion of the right to self-determimation is obviously the clearest case of nationalism and Christianity being easily conciliated, of course. But I believe that is not what you are asking about.