Part of St. Augustine of Hippo’s enduring legacy is the supposed dialectic between sacred and profane, between the transcendent and the secular. This emerged from Augustine’s writing of the City of God and the distinction between the city of man and the city of God and the unfolding of the two cities to their respective ends. The hyper monists, on either side, have argued that there is not intermixing of the two in the plane of the saeculum: That the holy city exists outside the saeculum though, in appearance, seems to be tied to it; that the earthly city is that which situates itself in the saeculum, and, therefore, is guided by “secular” law.
The saeculum, in Latin, means “of the age.” Augustine believed that the time between now and the Second Coming was the saeculum, the long age which will go on onto the final sorting out of the wheat and tares. I do not wish to provide a summary of the exhaustive scholarship on the relationship between Christianity and the birth of the “secular,” other than to refer accessible works by Larry Siedentop (Inventing the Individual) and R.A. Markus (Saeculum and Christianity & the Secular) on the matter for starters. What is undeniable is that there is, as James Wetzel has said, a “tangle of two cities” in this age and that there are two cities moving to their respective ends.
What, then, is the Christian’s relationship to the city of man, which he or she finds themselves in, and the heavenly Jerusalem, which he or she is pilgrimaging to? Augustine employs the city of God in two senses. In one sense the city of God is the Pilgrim Church on earth which, therefore, marks it as a “secular” institution. It has a beginning in time and will have an end with time. Then there is the City of God, the “Heavenly Jerusalem,” as he calls it from time to time. This city, properly speaking, exists outside of space and time. It is the eternal city, the final destination, the realization and fulfillment of all desires which the pilgrim city is laboriously struggling to.
Augustine, then, does create a nominal divide between the sacred and the secular. The sacred exists apart from the world while the secular exists wholly in the world. Christ, as the head of the church, mediates the two worlds, the two cities, the two citizenships. In this sense there is no divide between the sacred and the secular as the two meet either in the mythic theologies or civil theologies or all peoples, or, from Augustine’s own view, the proper mediation of the two through the true cult (the Christian Church headed by Christ).
Come Book XIX, Augustine makes clear that the role of the Church is to foster compromise between the divided cities of men to provide as much peace as possible (XIX.xii) The Church works for compromise because peace is something all humans seek (XIX.xvii). The peace, however imperfect, constructed in the world allows for people to pursue their ends, experience—however brief and incomplete—a taste of the heavenly peace of eternal Jerusalem, a taste of the true sacred which does touch down from time to time on earth. Because the Church is the mediator between the two worlds there is no dialectical thought—properly speaking—in Augustine’s work. Augustine is a unitive thinker. The dialectic seemingly embedded in Augustine’s work is not a dualism but a unitive and symbiotic relationship. The Church, for instance, calls the city of man to its higher calling—it’s unfallen and regenerative nature. The city of man, to be sure, can never realize itself as the heavenly Jerusalem, but the purpose of mediation is to provide triage on the battlefield so to speak. This is also the struggling task of the pilgrim Christian. To, at once, be at home in the world insofar as living in the world and making use of earthly goods, but also being estranged from the world in pilgrimage to one’s real home.
The question of whether Augustine accommodates a secular order or not is, in my estimation, to miss the mark on Augustine’s complex thought. The Church is a secular institution as it exists in time and will end with time. The City of God, however, is not a secular city for it exists in Heaven. The city of man is a secular city, for its existence is in time and will end with time too. The difference between Church and city of man is that the citizens of the heavenly city, all of whom are members of the Church in some fashion (though not all “visible” churchmen and churchwomen are citizens of that city), will have their continuation after time in the heavenly city.
There is no secular and sacred divide in Augustine because God works in this world and through time. God calls all to him, though not all will answer him. The mediating presence of the Lord, in and through his Church, ensures there is no “secular accommodation” but the other way around, a sacralization of the city of man. This is why the apostles marched to the ends of the earth. This is why God entered into space, time, and history in the person of Christ. This is why the Church exists in the world. It is not to condemn the world but to redeem the world.
Present outlooks on the secular-sacred divide from Augustine’s pen is tainted by the dialectical thought of Plotinus (who, admittedly, was an influence on Augustine), Hegel, and modern consciousness. It is hard for many moderns to wrap their head around sacramentality and the sacred like those ancient forebears, pagans and Christians alike who had no notion of the “secular” as we have today. Gibbon was wrong, as all ancient historians know, and as Augustine shows throughout City of God, that the pagans had a secularity to them in the same sense that we have today. Rome was hardly the secular world imagined by Gibbon and his ilk but was a deeply “sacred” city with its gods, sacred history and mythology, and understanding of divine mandate.
It is the emergence of Christianity that did not create the secular-sacred divide as much as it was a “this is not sacred enough.” There is only sacred history unfolding in Augustine’s mind. But that sacred history has a tragic element to it. We confuse the tragic sacred: The Fall of Man, Cain murdering Abel, the construction of Babel, and so on, as “secular.” As R.A. Markus wrote, “Augustine’s theology rejected the dichotomy of the sacred and profane…Sacred and profane, for him, interpenetrate in the saeculum.” It is the tragic sacred that people do not like. In rejecting the tragic sacred the “secular” was invented which divorced the tragic with the sacred and subsequently led to the “sacred-profane” dichotomy which many erroneously read back onto Augustine’s mighty pages.
 R.A. Markus, Saeculum: History and Society in the Theology of St. Augustine (New York: Cambridge University Press, 1988; 1970), p. 122.
You can read some of my professionally published writings on Augustine, especially relating to the City of God: