As you know, Augustine's most famous work is titled:
Dē cīvitāte Deī contrā pāgānōs
Literally, "regarding the city of God; against the pagans".
Starting a title with dē ("regarding") was a long-standing tradition, and we still see its remnants nowadays in works like On the Origin of Species; back before titles were really a thing, it was a way of saying what a text was about. The noun following dē is then in the ablative case: cīvitāte instead of cīvitas.
So if you're going for a title, you'd start with the same Dē cīvitāte Deī. The change would come after that.
To put it concisely, I'd use a passive participle next: …aedificātā ā pāgānīs "…established by [the existence of] the pagans". The verb here is a standard one for "to establish": it's the same one used in 1 Corinthians 8:1 and 14:4.
All together, that would be:
Dē cīvitāte Deī aedificātā ā pāgānīs
Regarding the city of God, established by the pagans