What follows is from Paul Johnson's "History of Christianity":
What Augustine absorbed in Ambrosian Milan, what he brought back to Africa, and what he opposed to Donatist particularism, was the new sense of the universality of the Church which the Constantine revolution had made possible. In Milan, Augustine had seen the Church, through the person of a shrewd and magisterial prelate, helping to run an empire. His creative mind leapt ahead to draw conclusions and outline possibilities. In Milan the Church was already behaving like an international organization; it would soon be universal. It was already coextensive with the empire; it would ultimately be coextensive with humanity, and thus impervious to political change and the vicissitudes of fortune. This was God's plan. Augustine had a historical view of human development. There were six ages: man was now living in the last, between the first and second comings of Christ, when Christianity would gradually envelop the world, as preparation for the final and seventh age. Against the background of this concept, the Donatists seemed ridiculously petty. They had grasped the seriousness of Christianity. But, by worrying about what particular bishops had done at a particular time and in a particular place, they had lost sight of the enormous, objective scale of the faith, its application to all places, times, situations. "The clouds roll with thunder," Augustine wrote, "that the House of the Lord shall be built throughout the earth; and these frogs sit in their marsh and croak -- 'We are the only Christians!'" Moreover, the Donatists had got the wrong notion of the world. Because of their obsession with their own limited local predicament and history, they saw the world as hostile and themselves as an alternative to society. But the world was there to be captured; and Christianity was not the anti-society -- it was society. Led by the elect, its duty was to transform, absorb and perfect all existing bonds of human relations, all human activities and institutions, to regularize and codify and elevate every aspect of life. Here was the germ of the medieval idea of a total society, with the church permeating everything. Was she not the Mother of All? "It is You," he wrote, "Who make wives subject to their husbands ... you set husbands over their wives; join sons to their parents by a freely-granted slavery, and set parents above their sons in a pious domination. You link brothers to each other by religious bonds tighter than blood …. You teach slaves to be loyal to their masters, masters to be more inclined to persuade than to punish. You link citizens to citizen, nation to nation, you bind all men together in remembrance of their first parents, not just by social bonds but by common kinship. You teach kings to rule for the benefit of their people, and warn the peoples to be subservient to their kings."
But the idea of a total Christian society necessarily included the idea of a compulsory society. People could not choose to belong or not to belong. That included the Donatists. Augustine did not shrink from the logic of his position. Indeed, to the problem of coercing the Donatists he brought much of their own steely resolution and certitude, the fanaticism they themselves displayed, and the willingness to use violence in a spiritual cause. To internationalize Africa, he employed African methods -- plus, of course, imperial military technology. When Augustine became a bishop in the mid-390's, the Donatist church was huge, flourishing wealthy and deeply rooted. Even after a long bout of imperial persecution, inspired by Augustine, the Donatists were still able to produce nearly 300 bishops for the final attempt at compromise at Carthage in 411. Thereafter, in the course of the two decades before Vandals overran the littoral, the back of the Donatist church was broken by force. Its upper-class supporters joined the establishment. Many of its rank and file were driven into outlawry and brigandage. There were many cases of mass suicide.
Augustine watched the process dry-eyed. Of course the times were horrific. The late empire was a totalitarian state, in some ways an oriental despotism. Antinomial elements were punished with massive force. State torture, supposedly used only in serious cases such as treason, was in fact employed whenever the State willed. Jerome describes horrible tortures inflicted on a woman accused of adultery. A vestal virgin who broke her vows might be flogged, then buried alive. The state prisons were equipped with the emulous, or rack; and a variety of devices including unci, for laceration, red-hot plates and whips loaded with lead. Ammianus gives many instances. And the State, to enforce uniformity, employed a large and venal force of secret policemen dressed as civilians, and informers, or delators. Much of the terminology of the late-imperial police system passed into the language of European enforcement, through the Latin phrases of the Inquisition. Augustine was the conduit from the ancient world. Why not? he would ask. If the State used such methods for its own miserable purposes, was not the Church entitled to do the same and more for its own far greater ones? He not only accepted, he became the theorist of, persecution; and his defences were later to be those on which all defences of the Inquisition rested.
We must not imagine that Augustine was necessarily a cruel man. Like many later inquisitors, he disliked unnecessary violence and refinements of torture. He thought heretics should be examined "not by stretching them on the rack, not by scorching them with flames or furrowing their flesh within claws, but by beating them with rods". He deplored, too, the dishonesty of using paid informers and agents provocateurs. But he insisted that the use of force in the pursuit of Christian unity, and indeed the total religious conformity, was necessary, efficacious, and wholly justified. He admitted he had changed his mind on this point. He wrote to a Donatist friend that he had seen his own town, originally Donatist, "brought over to the Catholic unity by fear of the imperial edicts". That had convinced him. In fact heretics in their hearts welcomed persecution: they would say "fear made us become earnest to examine the truth…the stimulus of fear startled us from our negligence". And then, this was Christ's own way. Had not he, "by great violence", "coerced" Paul into Christianity? Was not this the meaning from the text of Luke, 14:23: "Compel them to come in"? It was Augustine who first drew attention to this, and a number of other convenient texts, to be paraded through the centuries by the Christian apologists of force. He also had the inquisitorial emphasis: "The necessity for harshness is greater in the investigation, than in the infliction, of punishment"; and again: "…it is generally necessary to use more rigor in making inquisition, so that when the crime has been brought to light, there may be scope for displaying clemency." For the first time, too, he used the analogy with the state, indeed appealed to the orthodoxy of the State, in necessary and perpetual alliance with the Church in the extirpation of dissidents. The Church unearthed, the State castigated. The key word was disciplina -- very frequent in his writings. If discipline were removed, there would be chaos: "Take away the barriers created by the laws, and men's brazen capacity to do harm, their urge to self-indulgence, would rage to the full. No king in his kingdom, no general with his troops, no husband with his wife, no father with his son, could attempt to put a stop, by any threats or punishments, to the freedom and the sheer, sweet taste of sinning."
Here, first articulated, is the appeal of the persecuting Church to all the authoritarian elements in society, indeed in human nature. Nor did Augustine operate solely at the intellectual level. He was a leading bishop, working actively with the state in the enforcement of imperial uniformity. We have a vignette of him Carthage in 399, when imperial agents arrived to close down pagan shrines, preaching to excited mobs: "Down with the Roman gods!" Perhaps more sinister is Augustine's contact with authoritarian elements in Spain, already a centre of Christian rigorism and orthodox violence. There, in 385, the Bishop of Avila, Priscillian, a notable ascetic and preacher, had been accused of gnosticism, Manicheism and moral depravity, had been indicted under the imperial law of witchcraft, tried at Bordeaux, and brought to the imperial court at Trier. There, under torture, he and his companions confessed they had studied obscene doctrines, held meetings with depraved women at night, and prayed naked. Despite the protests of a leading Gaulish bishop, Martin of Tours, they were executed -- the first instance we have both of the slaughter of "heretics" and of witch-hunting under the Christian auspices. The episode aroused indignation, notably that of Ambrose, and provoked a reaction. But it did not end religious persecution in Spain; on the contrary, it was the beginning. Spain was already staging pogroms of Jews by the time Augustine became a bishop. And twenty years later we find him in correspondence with ferocious Spanish heresy-hunter, Paul Orosius, about the met means of winkling out heretics not only in Spain but at the other end of the Mediterranean in Palestine.