We turn now to chapter six — not the final chapter of the book (there is one more, and several appendices), but the final one dealing with purely historical matters, and the last we shall cover for the foreseeable future.
The Church and Political Power
Augustine’s mature reflections on “history, society and the Church”  may be described as achieving a kind of secularization via “a synthesis of three themes:” (1) the homogenization of history as all outside of “sacred” history, (2) the referring of state and social institutions to worldly ends only, and (3) the equating of the social existence of the Church to that of any other worldly society. Previous chapters have explored these themes. As Markus puts it,
These three strands together constitute what we may call a theology of the saeculum […], the whole stretch of time in which the two cities [of God and of earth] are ‘inextricably intertwined’; [the saeculum] is the sphere of human living, history, society and its institutions, characterised by the fact that in it the ultimate eschatological oppositions, though present, are not discernible […]. 
Augustine’s preparedness “to endorse religious coercion by the authorities of the state” , however, against the Donatists’ “insistence on religious freedom”, creates a problem: was Augustine inconsistent in his teachings about the blended saeculum? –or does this “readiness” for state-enforced orthodoxy falsify the interpretation of Augustine’s thought on the saeculum expounded in the previous five chapters? The possibility that “the fundamentals” of Augustine’s reflections on the saeculum might be consistent with his “attitude to religious coercion”  needs to be considered, as “there is an unresolved tension between Augustine’s mature theology of the saeculum and his views on coercion.” -
The African bishops were willing to solicit state power to abolish paganism and uproot idolatry.  In A.D. 399-401, Augustine still held to the widespread enthusiasm of the ideology of tempora christiana, seeing the Empire as the “vehicle of divine purpose in history”, fulfilling the Old Testament promises, and “subjugating [the] peoples to the worship of Christ.” This ideology provided “a fully fledged justification for religious coercion” of pagans.
Donatists were a different story. At this time, the African Catholic bishops were still engaged in efforts to persuade them peacefully. Even at the African Catholic Council of 403, they had only “invoked the support of the Proconsul”  to provide them with the archival material they needed to address Donatist historical claims of betrayal directly in public debate. The Donatists refused public debate. Tensions rose, and in the Council in 404, the Catholic bishops asked for repressive measures to be taken against the Donatists, implemented in the 405 “Edict of Unity”. “This inaugurated a sustained attempt by the government to repress the schism.” Augustine, however, “did not agree with the [A.D. 404] policy of coercion”. He was worried about “feigned” conversions if forced compulsion were to replace reasoned persuasion. Considering the matter as a pastor of his Church, he felt that the state measures against paganism had already brought about “low standards of popular Christianity”  via the “hypocrisy” of the subsequent conversions of coerced pagans. These resulted in merely “outward conformity”. Augustine feared that coercing the Donatists would result in a “worse corruption of standards”, yet seeing the results of the state measures against Donatism, Augustine’s fears evaporated.
How was this a possible stance for Augustine after A.D. 410, by which point he had dropped his support of the tempora christiana ideology enamored with the Theodosian establishment? He had abandoned the theory justifying coercion, but nonetheless his support for it remained.  How? The key to his support is to be found in his theory of pastoral discipline.  In it, “[h]arshness, fear, threats, coercion are not […] rejected on principle” as pastoral tools, though they are to “be kept in reserve”. Eventually, seeing the results of former Donatists being successfully absorbed by his flock, and the gratitude of some former Donatists for having been forced, Augustine “overcame his reluctance to sanction large-scale coercion”, which merely extended his sense of the appropriate “application of pastoral severitas.”  Violence, after all, must be “controlled by the public authority”, and this division between churches –or, rather, within the Church– was causing violence. Augustine eventually likened inter-Christian “religious coercion” to “medicine administered to an unwilling patient for his own good”. Even God “scourges those whom He loves.” State coercion forced them to do “what they would have wanted to do had they known better.” 
This pastoral element is what allowed Augustine to entertain the idea of state coercion in the case of the Christian Donatists, who already recognized the divinity of Christ. Granted, coercing pagans was futile, for it commanded them to “accept a truth to which they were blind”, but the Donatists were merely recalled “to the flock to which they rightfully belonged and from which they had strayed”, so that this coercion was “part of the Church’s pastoral activity among its own flock”, rather than violence against outsiders, such as coercing pagans. The state is not a persecutor here, but “persecuting [the Donatists’] persecutor, that is, [their] error”. (Nor is the “state” acting as the “state” here, as we shall see later.) Divine disciplina employs external means of bringing about inner moral transformation.
Especially after his extended reflections on the writings of Paul of Tarsus, Augustine considered “freedom of choice less and less as something incompatible with constraint and fear.”  The parable of the wedding feast in Luke, where the host of the feast commands his servants to “compel [those outside] to come in” (Luke 14:23), is taken as precedent:
Let constraint be found outside: the will is born within. 
Divine discipline is necessary for transformation [GDS — strangely at odds with Augustine’s earlier reported position on divine and eternal law as unable to reform the inner man]; evil habits hold us in bondage, and they are not “external to the personality” in such a way that allows them to be easily discarded in conversion and baptism. These evil habits were the walls around schismatic communities, capable of being broken by harsh pastoral disciplina.  This is not only pastoral for those separated Christians, but even consistent with Augustine’s later soteriology: after all, he “could equate God’s action on human will with external impingement”. 
In the end, and more fateful for later Christian history, it is the “theme” of the “Christian ruler” that rubs up against “the central problem” here:
the Christian ruler or magistrate can be thought of either as a member of the Church, or as part of a system of governmental institutions; either as a servant of God or of the res publica. 
In his earliest writings, Augustine saw the Church “as the guardian of all established order and of all proper human relationships” in the Greek manner we saw in chapter four: “[t]he tasks and interests of Church and State necessarily coincided in the person of the ruler.”  This was “strengthened by the Eusebian type of theology” where
[t]he Empire was the instrument of a divine purpose, the Emperor a quasi-messianic figure with a transcendent mission; Church and State were only provisionally distinct aspects of a single Christian ‘polity’. 
Defining the respective spheres of these two was “by-passed, for the two were, at root, identical.” Augustine abandoned this ideology, rejecting the idea that any given polity might be Christian, but still spoke “of Christian rulers and officials owing specific service to God” in their public offices. Writing “to the Roman commander Boniface” in 417 [Epistle 185], Augustine states that this service entails
commanding what is good, prohibiting what is wicked, not only in matters pertaining to social relations between  men, but also in those pertaining to divine religion. [147-148]
[GDS — This is not the merely punitive role mentioned in earlier chapters, presumably because of the overlap in roles, ecclesial and civil, as we see below.] There is “no suggestion” here that state authority has a sphere “restricted to ‘temporal’ matters.” Augustine’s teaching on the saeculum is not inconsistent with his notion of this Christian civic service against the Donatists, however, for he advises him that
when you act, the Church acts, for whose sake and as whose son you act. [Epistle 173] 
Augustine never considered Christians in public service as part of a state machine. “He thought of them as members of the Church. Through them, it is the Church that ‘uses power’.”
Augustine does not think of Christians in public office as representatives of the state, but as individuals. In his thought, there was a “perpetual liability [for] […] the state to dissolve into a kind of atomistic personalism.”  Individuals are the atoms of empirical cities, and might belong to either eschatological city.  This atomism allowed him to critique both “pagan Roman” and “Christian empire” ideologies; this atomistic aggregate is itself the blended saeculum. No concrete social group was capable of a collective assessment. Augustine “by-pass[ed]” the “collective, institutional character [of every society] […] and […broke] them down […] into individuals with their personal loyalties to different, and very mixed, ultimate values” in order to allow the differences between people to remain in a shared space of proximate loves. As we noted before, he did not create a third space to mediate between the differences, but conceived of the differences as resolving only partially in temporal loves of very different individuals.
Thus, Augustine’s advocacy of religious coercion is not a coercion of the state, but “by the Church”.  The “‘state’ tended to dissolve in his hands.”  Indeed,
the fact that [Augustine] did not think of [coercion] in terms of the state, but in terms of individual members of the Church, who held secular office, disguised from Augustine the acute tension between his consent to coercion and the implications of his theology of history and society. 
The very atomistic means of framing a pluralist saeculum enabled a Christian individual in office to act ex ecclesia, and to enact pastoral discipline via the tools of the state as a son of the Church, without parsing out the respective spheres of each.
Markus simply names it “[t]his horrible doctrine” , though John Dillon, in his review, puts it more mildly: “[a]n unlovely theory, but coherent enough.”