But then, sailing from Carthage, came Donatus. A man less committed to compromise it would have been hard to imagine. Even before his election as bishop, he and his followers had taken the momentous step of complaining to Constantine about Caecilian, and demanding his deposition. The emperor, puzzled at finding such divisions among Christians, nevertheless permitted Donatus to make his case before a panel of bishops in Rome – who promptly found against him. Donatus appealed; and again had his case rejected. Still he pestered Constantine with complaints. When, in 316, he managed to slip the guards who had been placed on him by the weary emperor, and make it back to Africa, his escape only confirmed Constantine’s dark opinion of the bishop’s contumacy. Henceforward, in the bitter clash between Donatists and Caecilianists, it was the latter who would have the might of the Roman state on their side. ‘What business has the emperor with the Church?’56 Donatus’ question, suffused with outrage and resentment though it might be, was in truth rhetorical. Constantine, no less than any bishop, believed himself entrusted with a heavenly mission to uphold the unity of the Christian people. The tradition embodied by Donatus, the conviction that the Church was most pleasing to God when its members repudiated those of their fellows who had fallen into sin, perplexed and infuriated him. ‘Such squabbles and altercations,’ he fretted, ‘may perhaps provoke the highest deity not only against the human race, but against myself.’57 By giving Caecilian his support, Constantine was assuring bishops across the empire that, provided only that they assented to the emperor’s desire for a unified Church, they too could rely on his backing. Donatus, meanwhile, had to live with the painful knowledge that his claim to the leadership of Christians in Africa was accepted by few beyond the limits of the province. It was the followers of Caecilius, in the eyes of the world, who were the authentic ‘Catholics’; those of Donatus were ‘Donatists’ still.
Yet if bishops had to scramble to adjust to the new circumstances heralded by Constantine’s victory at the Milvian Bridge, so too did the emperor himself. Fully committed as he was to understanding what it meant to be a servant of Christ, he found himself embarked on a steep learning curve. His altercations with Donatus had brought home to him just what he faced in the Church: an organisation over which, despite his rule of the world, he had no formal control whatsoever. Unlike the priests who traditionally had mediated between Rome and the heavens, bishops did not bother themselves with rites in which he, as the heir of Augustus, could take the lead. Instead, to Constantine’s intense frustration, they insisted on squabbling over issues that seemed better suited to philosophers. In 324, alerted to the inveterate taste of theologians in Alexandria for debating the nature of Christ, he did not bother to conceal his impatience. ‘When all this subtle wrangling of yours is over questions of little or no significance, why worry about harmonising your views? Why not instead consign your differences to the secret custody of your own minds and thoughts?’58 Yet it was dawning on Constantine that these questions might be naïve. The issues of who Christ had truly been, in what way he could have been both human and divine, and how the Trinity was best defined, were hardly idle ones, after all. How could God properly be worshipped, and his approval for Rome’s rule of the world thereby be assured, if his very nature was in dispute? Constantine’s predecessors, with their attempts to appease the heavens by offering them their ancient dues of sacrifices and honours, had grievously misunderstood what was required of an emperor. ‘It matters not how you worship, but what you worship.’59 True religio, Constantine was coming to understand, was a matter less of ritual, less of splashing altars with blood or fumigating them with incense, than of correct belief.
A decisive moment. In 325, only a year after he had been advising rival theologians to resolve their differences, Constantine summoned bishops from across the empire, and even beyond, to a council. Its ambition was fittingly imperious: to settle on a statement of belief, a creed, that churches everywhere could then uphold. Canons, measures to prescribe the behaviour of the faithful, were to be defined as well. The venue for this great project, the city of Nicaea in the north-west of Asia Minor, was pointedly not a Christian powerbase. Constantine himself, ‘clothed in raiment which blazed as though with rays of light’,60 welcomed his guests with a display that mingled graciousness with just the faintest hint of menace. When at length, after an entire month of debate, a creed was finally settled upon, and twenty canons drawn up, those few delegates who refused to accept them were formally banished. The fusion of theology with Roman bureaucracy at its most controlling resulted in an innovation never before attempted: a declaration of belief that proclaimed itself universal. The sheer number of delegates, drawn from locations ranging from Mesopotamia to Britain, gave to their deliberations a weight that no single bishop or theologian could hope to rival. For the first time, orthodoxy possessed what even the genius of Origen had struggled to provide: a definition of the Christian god that could be used to measure heresy with precision. In time, weighed in the balance against the Nicaean Creed, Origen’s own formulations on the nature of the Trinity would themselves be condemned as heretical. A new formulation, written, as Origen’s had been, in the language of philosophy, declared the Father and the Son to be homoousios: ‘of one substance’. Christ, so the Nicaean Creed proclaimed, was ‘the only Son of God, eternally begotten of the Father, God from God, Light from Light, true God from true God, begotten not made’. Never before had a committee authored phrases so far-reaching in their impact. The long struggle of Christians to articulate the paradox that lay at the heart of their faith, to define how a man tortured to death on a cross could also have been divine, had at last attained an enduring resolution. A creed that still, many centuries after it was written, would continue to join otherwise divided churches, and give substance to the ideal of a single Christian people, had more than met Constantine’s hopes for his council. Only a seasoned imperial administrator could possibly have pulled it off. A century after Caracalla’s grant of citizenship to the entire Roman world, Constantine had hit upon a momentous discovery: that the surest way to join a people as one was to unite them not in common rituals, but in a common belief.