Yep. That's the City of God. And that's a multicultural city. Augustine was advocating multiculturalism. So Augustine can't serve as an authority for the Williamson's pro-nationalist views.
Williamson recognizes this problem and tries to get out of it by spiritualizing this multiculturalism, viz., A closer reading shows, however, that the “citizens” are called “out” in a spiritual rather than a physical sense.
But this is just obfuscation.
I agree with you, Codeword. St Augustine's City is clearly a metaphore for the Western Civilization, which has a common language philosophically but not linguistically, and the need for genetic cohesion is pure invention of Williamson's.