Nation is a natural institution just like the family. It is distinct from the State which is the civic government.

The State can govern a single nation (say Poland) or many nations (the Holy Roman Empire).

Religious faith is not a substitute for your nation. Thus, you belong to a particular family, a particular nation, and you profess a particular religion. You live in a State.

These are all different things.

Christendom did not obliterate nation and replace it with Catholicism. Rather, it allowed different ethnic (national) groups to exist without trying to homogenize them.

It is the fruit of the Enlightenment that states have embarked on homogenization of the peoples they rule. For example France crushed all dialects apart from the official French.