The Ancient Romans spoke of ‘Peoples’ and ‘Nations’. They saw traditions making a group into a people and distinguishing people (and, sometimes, ‘nations’) from each other. Different peoples (distinguished from each other on the basis of ancestral traditions) could live together in a nation and a nation itself could host multiple peoples and traditions. Consequently, they did not see Christians as a people because they did not have ancestral traditions to follow.
The Christians redefined what it is to be a people: they became a ‘communitas’ (a community) because of their ‘religio’. The Christians became a community because they were united “in God” and “in Christ”. They were united too in their “search for God”, which was what “conversio” (‘conversion’) meant to them. Christians were a ‘people’ now, a community that came into existence by their ‘unity in Christ’. The idea that individuals are “united” in “God and Christ” is what brings about a community and thus a people. This is a very Christian idea, which is based on the thought that individuals are united only in God. When secularized, it takes the shape of the idea that “a people” have a “common religion”. Or that every people is a people (or becomes one) because of religion.
It is this direction that one will have to go. I am working this idea out (every now and then) in my attempt to build a foundation for a theory of comparative politics, where I contrast these Christain ideas with the Indian ones. II One is the meaning of ‘nation-state’, which is also abbreviated in colloquial use as ‘nation’. This picks out individual countries with physical borders that have sovereign states internally: U.S., India, Saudi Arabia, Yemen, etc are such nation-states.
The second idea, which has a venerable history of more than 2000 years, is one which sees ‘nation’ as a ‘people’. In that sense the Jews are a ‘nation’ as are the Christians (as they saw themselves earlier and many, even now) and Muslims. Both the Muslims and the British saw a ‘Hindu’ nation in India as well.