I assume you are referring to terminology from Augustine of Hippo's City of God

Writing in the early fifth century, when the Western Roman Empire was going into terminal decline, Augustine sought to explain the current state of Rome as part of a theology of history, based on the scriptural teachings concerning God’s judgement on Babylon and Jerusalem. Envisaging history as a tale of two cities, the earthly and the heavenly.

Augustine reassured Christians, stating that Rome had NOT fallen because the people had abandoned the Roman ‘Fortuna’ (old pagan practices) in favour of Eastern prophecy, but BECAUSE Rome itself had become ‘the great Western Babylon’; the New Testament metaphor for corruption.

So yes, when Augustine describes an Earthly City, or the City of Man, he was primarily talking about Rome. He was saying that although the earthly seat of Christian officialdom, Rome, had become corrupt and unstable it was the heavenly City of God that would ultimately win out

Basically urging citizens of the crumbling Roman Empire to put their faith in the spiritual rather than the material