The Church’s pre-eminent theologian St. Thomas Aquinas teaches in the Summa Theologica  that relations with foreigners can be either peaceful or hostile according to the Moral Law. If certain nations had a close relationship then citizenship could be obtained after the third generation of living peacefully in a compatible nation; whereas others with whom their relations were hostile were never to be admitted to citizenship.

The Angelic Doctor further teaches in De Regno, quoting Aristotle’s Politics, that in general an influx of foreigners is particularly harmful to civil life and civic customs.