How was nationalism expressed in the late middle ages? why was this a threat to the old ideal of a unified "christendom"?

Cultural unity disintegrated in the late middle ages. The feudal social structure lessened its hold on the people. Added to this was the effect of the French Revolution that spiked up the loyalty of the people to the land instead of the prevailing society and culture. The establishment of governments united the people more giving them a sense of unity that became a threat to the then prevailing power of Christianity in Europe. Christianity during the middle ages was the force that united the continent of Europe. It represented the voice of power that dominated even the different kingdoms.