The city of God of Augustine, which he began writing at an age of 59 and worked on it, for much of the next 14 years. Augustine displays tremendous scholarship, employing the writings of Paganism's greatest historians and philosophers in his case against their religious claims. The result is a giant literary, philosophical, historical, theological and exegetical work. It's a demanding work, but it can be read without any specialized historical knowledge.
Augustine reflects deeply here on human nature and the meaning of eternal life and eternal punishment, within an explication of the 'meaning' of history. He writes of all human history as a single narrative. This also a work of Biblical exegesis, as Augustine treats Scripture as a historical document.
The impetus for the beginning of this vast work (and its recurring focus) was the charge of Pagans (polytheists) that Christianity was responsible for the decay and demise of the Roman Empire. The charge put forward the claim that the prosperity and social stability of the state was dependant upon polytheistic worship. In response, Augustine arrays several lines of argument, rebutting the assumed 'goodness' of the Pagan state, as such, and detailing the ethical/moral and logical failings of Paganism. Augustine displays tremendous scholarship, employing the writings of Paganism's greatest historians and philosophers in his case against their religious claims. The result is a giant literary, philosophical, historical, theological and exegetical work.
The book contains the parallel histories of what Augustine terms the City of God and the City of Man, both descended from Adam. The City of Man is founded on murder (specifically fratricide, the murder of a brother, viz. Cain and Abel, Romulus and Remus). The City of Man has been deceived and debased, fallen under the sway of pagan gods, which appear to be either demons or, at best indifferent or benign spirits that are mistakenly worshipped.
The City of God, on the other hand, is a pilgrim on this earth, toiling here in the joyous expectation of final salvation in God's Kingdom. Thus there are two 'cities', two loves, two ways to understand the big questions of existence, two destinations. Among the many philosophical (and historical) passages of interest are Augustine's general recountings of the history and development of Italian and Ionian philosophy, in Book VIII, particularly as regards ethics, theology, physics and cosmography. Augustine wants to explain the ways of God to man, but he does this from some humility, expressing his speculation in doubt. City of God also shows Augustine to be interested in the goods of Greek and Roman philosophy and rhetoric and in purging the negative elements of these while and Christian revelation. He's always intent on removing the possibility of gnostic/Manichaean distortions of Christian texts, such as St. Paul's admonitions not to "live according to the flesh" but rather "according to the spirit." Augustine is clear that this does not mean disdain for the body, but that one should refuse to live according to human ways, and consent to live by God's will.
For the reader whose serious interest is Christian theology and scriptural exegetics, Augustine needs no introduction. It would be fair to describe him as the most influential human voice of the Christian faith.