Christianity by the Century: Fourth Century CE (300–400)

When tracing the history of Christianity, it is tempting to focus on the first century—the lifetime of Jesus and the earliest followers. Yet remembering that Jesus never was a Christian—he was always a Jew—it becomes clear that the fourth century, rather than the first, marks the most transformative period in Christian history. During this century, Christianity shifted decisively from a persecuted minority movement into an imperial religion, reshaping Christian identity, authority, and belief in lasting ways.

If the third century embedded suffering and martyrdom into the meaning of Christian identity, the fourth century introduced something entirely new: power. Christianity moved rapidly from a persecuted minority to a legally recognized and increasingly influential religion. That transformation raised urgent questions about who had the authority to define what it meant to be “Christian.” Recognition brought resources, protection, and prestige—and it also attracted new participants whose commitments were no longer shaped by persecution. As Christianity gained power, struggles over control, leadership, and orthodoxy intensified.

Against the backdrop of centuries of persecution, several developments were taking place that set the stage for the profound shift in the 4th century.  Christianity, such as it was, had been developing its first forms of true structure through their bishops.  They had provided various levels of local leadership in worship, increasingly acting as intermediaries between Christian communities and the Roman state.  Their care for the poor and vulnerable fostered greater dependency, which strengthened their authority and created a sense of hierarchy. Ultimately, they adopted Roman legal frameworks and administrative structures, gradually accumulating wealth, property, and influence.

Constantine and the End of Christian Persecution

Constantine converted to Christianity following a vision of a cross in the sky he had seen just before the Battle of the Milvian Bridge in 312. He believed this sign guaranteed his victory, and along with his desire to unify the Roman Empire under a single, strong faith, this pivotal event led him to favor Christianity. In 313 he and Licinius issued the Edict of Milan, which legalized Christianity and ended state-sponsored persecution. For Christian communities long shaped by suffering and resistance, this change was nothing short of revolutionary.

To further his goal of unification under a single faith, Constantine called bishops from all the Roman provinces to a council at his summer palace in Nicaea in 325. By the time he convened them, bishops had already been emerging for over a century as key figures in Christian life. They had been gradually accumulating wealth, property, and influence, and by the early fourth century, bishops were well positioned to function as imperial partners—natural intermediaries between Christian communities and the Roman state.

The Council of Nicaea

The very structure of the Council of Nicaea reveals how imperial priorities shaped early Christian orthodoxy. Only bishops were invited—male leaders who fit neatly into Roman administrative hierarchies. The emerging model mirrored imperial governance: one God, one faith, one bishop, just as there was one emperor and one law. Entire Christian communities were excluded from participation. Women leaders, prophets, teachers, Marcionites, Montanists, Valentinians, and enslaved visionaries—groups that likely represented a numerical majority of Christians—had no voice in defining doctrine.

Once authority became concentrated in the hands of bishops—now operating within imperial systems—doctrinal disagreement carried far greater stakes. Theological debates were no longer confined to local communities; they threatened ecclesiastical authority and imperial unity alike.

The most explosive theological conflict of the fourth century arose in Alexandria, where a priest named Arius clashed with his bishop, Alexander. The dispute centered on the nature of Christ—whether Jesus was fully divine, fully human, or a created being subordinate to God. This question mattered deeply, not only for theology but for authority. Whoever defined Christ’s nature effectively defined the structure of Christian leadership.

Arius argued that if Christ were fully divine and eternal, then Jesus could not truly serve as a model for human behavior. Alexander rejected this view, insisting that Christ shared the eternal essence of the Father. The controversy quickly spread beyond Alexandria, dividing Christian communities across the empire. Riots, expulsions, and political maneuvering accompanied theological debate, revealing how closely Christian belief and imperial power had become intertwined.

The Council of Nicaea produced a creed affirming Christ’s full divinity and condemning Arius, who was exiled. Yet the Nicene Creed did not end the conflict. After Constantine’s death in 337, his sons supported opposing theological factions, and imperial favor shifted repeatedly. Throughout the mid-fourth century, Arian bishops gained influence at court, overturned Nicene decisions, and then lost power again. Excommunication became a political weapon. Although Arian Christianity eventually declined, its persistence shows that fourth-century Christianity remained far from unified.

How Imperial Power Shaped Christian Orthodoxy

Understanding why Nicene Christianity ultimately triumphed requires attention to social and political factors as much as theological ones. The third-century emphasis on suffering and martyrdom had created communities that leaned on leadership capable of providing protection, resources, and advocacy.

At the same time, Christian theology began to mirror Roman social patterns. Christ was increasingly portrayed as the supreme ruler of a cosmic empire, reflecting the Roman ideal of the paterfamilias—the strict father whose authority was unquestioned. This model supported a centralized, male-dominated church structure and helped stabilize one-man rule, both imperial and ecclesiastical.

Legislating Orthodoxy: From Theology to Imperial Law

Once imperial favor was secured, orthodox leaders moved quickly to enforce boundaries. Church councils legislated exclusion with increasing precision. The Synod of Laodicea in the 360s prohibited women from approaching the altar. Bishops received tax exemptions, judicial authority, and control over church property. In 380, Emperor Theodosius issued the Edict of Thessalonica, declaring Nicene Christianity the official religion of the Roman Empire. Competing forms of Christianity were no longer merely incorrect; they were illegal.

Fourth-century church historians such as Eusebius framed this development as a story of unity and divine progress toward orthodoxy. Yet this narrative silenced large segments of Christian life. Precise doctrinal definitions became tools for enforcing loyalty to imperial Christianity. Only recently have historians recovered alternative accounts revealing the depth of early Christian diversity.

Priscillian of Avila and the Criminalization of Dissent

The story of Priscillian of Avila offers a striking example of how dissent was handled in the fourth century. Like Marcion, Mani, and Valentinus before him, Priscillian was a popular Christian teacher. He emphasized ascetic discipline, devotion to Scripture, care for the poor, and resistance to what he perceived as corruption within the church hierarchy. For many Christians, this was authentic discipleship.

Priscillian’s views resonated with diverse Christian movements throughout the Roman world, including Manichaean communities in the east and various gnostic traditions. Many shared a broadly dualistic outlook, viewing the material world as a distraction from spiritual truth and advocating rigorous ascetic practice as a path to spiritual growth.

Unlike increasingly institutional bishops, Priscillian turned primarily to sacred texts rather than ecclesiastical authority. This independence alarmed church leaders now backed by imperial power. Traditional methods of control—excommunication and denunciation—proved insufficient. A council at Saragossa in 380 attempted to suppress his influence, but a few orthodox bishops supported him and even ordained him as bishop.

Heresy, Violence, and the First Christian Execution

As Priscillian traveled across the empire defending himself, his case intersected with broader struggles over orthodoxy and power. Emperor Maximus convened a synod at Bordeaux aimed at condemning Manichaeism, but the vague wording of its anathemas encompassed Priscillian as well. Seeking political advantage, Maximus charged Priscillian with sorcery(!)—a civil crime punishable by death.

Martin of Tours protested that the accusations were baseless, warning against the use of state violence in theological disputes. But his objections failed. In 384, Priscillian was executed by beheading, becoming the first Christian executed by the state for heresy. Even those who disagreed with his teachings recognized the dangerous precedent: imperial authority now enforced Christian orthodoxy through violence.

By the end of the fourth century, collaboration between bishops and civil authorities had become normalized. This alliance between church and empire shaped Christian history for centuries, often at the expense of theological diversity and spiritual freedom.

Monasticism, Scripture, and the Spread of Fourth-Century Christianities

Yet the fourth century also brought valuable and creative developments. Monasticism emerged as an alternative Christian path emphasizing prayer, poverty, and service rather than power. The translation of biblical texts into Latin produced the Vulgate, which became foundational, especially in Western Christianity. Missionary efforts spread multiple forms of Christianity across Europe, North Africa, and Asia.

Although the Roman church labeled Arianism, Manichaeism, and Priscillianism as heresies, these movements continued to spread alongside orthodox Christianity. The fourth century did not produce a single, unified faith but a contested religious landscape filled with competing visions of what it meant to follow Jesus.

Conclusion

The fourth century stands as a decisive turning point in Christian history. Communities that had once defined faith through suffering and martyrdom now faced the challenges of legitimacy, authority, and power. Christianity gained legal protection, imperial backing, and institutional strength—but these gains came at a cost. Diversity narrowed, dissent was criminalized, and spiritual experimentation gave way to enforced orthodoxy.

What emerged was not a single, inevitable form of Christianity, but one shaped by empire: hierarchical, male-dominated, and aligned with Roman models of rule. The tensions forged during this century—between suffering and power, unity and plurality, conscience and control—have never fully disappeared. They continue to shape Christian institutions, theology, and identity long after the Roman Empire itself has fallen.

This is the ninth in a series of Bible and Beyond blog posts, Christian History by the Century, focusing on the key developments that influenced Christianity, century by century. Beginning with the fifth century BCE, we have traced historical events and ideas that influenced future generations of those who came to identify themselves as Christian.