The Roman Matrix: Constantine, Theodosius, and the Foundations of a Theopolitical State

The fourth century CE constitutes the decisive hinge upon which the Roman world turned from the classical past into the Byzantine millennium. This chapter contends that the foundational matrix of the Byzantine “unbroken realm”—that enduring synthesis of sacralized imperial authority and a politically enmeshed yet theologically assertive church—was not a product of the seventh-century crises but was deliberately forged in the reigns of Constantine the Great (306–337) and Theodosius I (379–395). It was in this period that the Roman imperial ideology, with its inherent claims to universal dominion and divine favor, was systematically fused with the theological constructs of Nicene Christianity, producing a theopolitical state wherein the emperor functioned as God’s sole vicegerent on earth and the ecclesiastical hierarchy was incorporated into the imperial administrative machine. This synthesis, however, was inherently paradoxical: it sacralized caesaropapist impulses while simultaneously creating the institutional space for ecclesiastical autonomy to push back, establishing a pattern of negotiation and conflict that would define Byzantine political theology for a thousand years. The military reorganizations of the late fourth and early fifth centuries, particularly the formal division between border troops (*limitanei*) and mobile field armies (*comitatenses*), further entrenched this model by linking military command directly to imperial authority and prefiguring the later thematic system’s fusion of civil and military power. Understanding this transformative century is thus essential for deciphering the Byzantine Empire’s self-conception as a *basileia* ordained by God, a *oikoumene* under one emperor, and a *politeia* where canon law and imperial legislation were inextricably intertwined.

I. Constantine’s “New Rome”: Capital, Cult, and the Ambiguities of Christian Imperial Authority

The accession of Constantine following his victory at the Milvian Bridge (312) did not immediately inaugurate a Christian empire; rather, it initiated a prolonged and deliberately ambiguous process of transformation. His famous vision of the *labarum* and the subsequent adoption of the Chi-Rho as a military standard have been endlessly debated, but what remains clear is that Constantine’s primary objective was the restoration of imperial unity and the stabilization of a fractured empire. Christianity offered a potent ideological tool for this purpose: a trans-regional, hierarchical, and monotheistic framework that could rival and supplant the traditional Roman cults while providing a divine sanction for imperial power. Yet Constantine’s relationship with the Christian church remained complex and instrumental, characterized by active intervention in doctrinal disputes, the assertion of imperial authority over ecclesiastical matters, and a personal piety that never fully renounced elements of the old religious order.

The founding of Constantinople (330) as a “New Rome” on the site of ancient Byzantium was the most tangible expression of this new theopolitical vision. More than a mere relocation of the imperial seat, it was a symbolic act that re-centered the Roman world in the Greek East while embedding Christian cosmology into the very fabric of the capital. Constantine spared no expense in adorning his new city with the spoils of pagan art—statues of Apollo, Artemis, and Athena were repurposed to decorate public spaces—yet he simultaneously endowed it with monumental Christian structures: the original Church of the Holy Apostles, the Church of the Peace (*Eirene*), and, most significantly, the first Great Church (*Magna Ecclesia*), later known as the Hagia Sophia. This deliberate juxtaposition was not syncretism in the traditional Roman sense but a declaration of Christian triumph within a Roman framework. The city’s dedication ceremonies, while incorporating traditional Roman rituals such as the *suovetaurilia* sacrifice, were reoriented toward a Christian understanding of imperial legitimacy. Constantinople was thus conceived as a *nova Roma* where the ancient Roman imperium was renewed through the lens of the new faith, a physical manifestation of the synthesis between Roman political theology and Christian eschatology. Its very topography—the imperial palace adjacent to the Great Church, the Hippodrome as a space for both imperial acclamation and popular Christian festivity—embodied the fusion of secular and sacred authority that would become Byzantine normality.

Constantine’s ecclesiastical policy was defined by a profound ambiguity. The Edict of Milan (313), issued jointly with Licinius, granted religious toleration to all, including Christians, and restored confiscated property. Yet this was not an act of Christian triumphalism but a pragmatic measure to secure divine favor for the empire by ending persecution. Constantine actively intervened in the theological controversies that threatened church unity, most notably the Arian dispute. By convening the Council of Nicaea (325)—the first ecumenical council—he asserted the emperor’s right to summon bishops, set the agenda, and enforce doctrinal decisions. The resulting Nicene Creed, with its homoousion formula, was as much a political document as a theological one, intended to impose a unified belief system that would support imperial stability. Constantine’s subsequent support for the semi-Arian party at the Council of Serdica (343) and his eventual baptism by the Arian bishop Eusebius of Nicomedia on his deathbed reveal the limits of his theological commitment. His famous statement to the bishops at Nicaea—“I am a bishop, ordained by God to oversee the external affairs of the Church”—encapsulates the caesaropapist impulse: the emperor as the ultimate arbiter of ecclesiastical order, responsible for the *externals* of religion (unity, property, public order) while leaving *internal* doctrine to the clergy, yet never fully relinquishing the right to intervene. This set the precedent for imperial *epimeleia* (care) of the church, a concept that would be refined by later emperors but whose roots lie in Constantine’s personal assumption of a role akin to a Christian *pontifex maximus* without ever formally claiming the title.

The material and institutional foundations laid by Constantine further cemented this fusion. He granted the church extensive legal privileges, including the right to receive bequests (*ius accepti*), exemption from certain taxes, and the establishment of a formal clergy (*clergy*) with state-subsidized salaries. The *patronatus* system, whereby the emperor became the primary patron of the church, created a relationship of dependency that would shape ecclesiastical politics for centuries. Most critically, Constantine initiated the practice of imperial involvement in the election of bishops, particularly in major sees like Alexandria, Antioch, and Jerusalem. While he did not directly appoint them, his influence—often exercised through the local secular governor—was decisive. This early politicization of the episcopacy meant that the highest church offices were inevitably entangled in imperial court politics, creating a class of bishops who were as much imperial functionaries as spiritual leaders. The career of Eusebius of Caesarea, Constantine’s panegyrist and chief theological advisor, exemplifies this new role: a bishop who saw no contradiction in praising the emperor as God’s appointed ruler and who helped craft an ideology of Christian empire that presented Constantine as a new David or Moses, a ruler chosen to implement God’s will on earth.

Yet the ambiguities of Constantine’s settlement were evident from the start. His retention of the title *pontifex maximus* (though some scholars debate its continuous use) and the continued presence of pagan symbols on his coinage—most notably the Sol Invictus “Unconquered Sun” with the inscription *SOLI INVICTO COMITI*—suggest a ruler who sought t