The City of God Against the Pagans (Latin: De Civitate Dei contra Paganos) is a major philosophical work written by the Latin Christian St Augustine of Hippo in the early 5th Century AD.
Written as a refutation of those who blamed Christianity for the sack of Rome in 410 AD, the work cast history as a cosmic struggle between the Earthly City and the City of God, with the latter destined to be victorious. It had profound effect on the worldview of the Latin West in the Middle Ages.
Free online texts
Bibliotheca Augustana: de Civitate Dei. Latin text, HTML format.
Christian Classics Ethereal Library: St. Augustine's City of God and Christian Doctrine, translated by Philip Schaff. Multiple formats.
Internet Archive: City of God Complete Vols 1 and 2. Translated by Dods (1871). Multiple formats.
Latin Library: Augustine of Hippo, Latin texts.
Georgetown University: Augustine of Hippo, by James J. O'Donnell.
History of Philosophy without any gaps: Heaven and Earth: Augustine’s City of God - podcast by philosopher Peter Adamson.
Stanford Encyclopedia of Philosophy: Saint Augustine, by Michael Mendelson.
Librivox: The City of God - public domain audiobook.
PhilPapers: Augustine - bibliography with open access option.
Wikipedia: The City of God (book).
Further reading at Tom's Learning Notes
Orosius: Seven Books of History Against the Pagans - A historical work by an acquaintance of Augustine with similar aims to The City of God.
Latin resources: Learn to read Latin texts in the original.
Bloom's Western Canon: The City of God is listed.