How did Augustine’s idea of the Two Cities oppose a future earthly millennium?

In his treatise The City of God, Augustine opposed the expectation of a future earthly millennium by presenting history as a coexistence of two distinct realms: the heavenly city and the terrestrial city (or City of Man).12

He argued that millennial perfection could not be achieved within the terrestrial world, where good and evil remain inextricably intermingled until the end of time. By defining the millennium as a "realized" state that had already begun with the establishment of the church, Augustine decoupled the concept from future, apocalyptic expectations. In this view, the "first resurrection" occurs continuously through the sacrament of baptism, and the peace of the heavenly city is a spiritual reality rather than a future political or social utopia on earth.123

This theological framework allowed Augustine to reject the notion that history’s end could be predicted through contemporary events, effectively "desanctifying" history and shifting the focus from imminent supernatural intervention to a life of faith within the saeculum (the present age).1