On the other hand, there are universalist tendencies in many religions that actively undermine nationalism, because people develop sympathies with members of the same faith among other nations and peoples rather than with their own countrymen. A good example of this are many liberal American Catholics who want to inundate the US with as many Latin American immigrants as possible, and many Evangelicals who want to bring in lots of Third World people as potential converts recruited to their Churches. On the national question, religious organizations in the US have been consistently unreliable.
I would say the inexact opposite: nationalism is good when Christian, bad otherwise. Indeed, peoples who are not Christian are not really a people: "[you] in time past were not a people: but are now the people of God." (1 Peter 2:10).
Mass replacements of one people with another has nothing to do with Christianity.