Christianity becomes the Official Religion of the Empire

The separation of Church and State in the Roman Empire is a major evolution in human society. It represents the semi-autonomous nature and interrelationship of two social spheres: culture and governance. This evolution enabled the development of parallel social institutions, not under the same authority.

In 285/286 c.e., Emperor Diocletian had divided the unwieldy Empire into Eastern and Western administrative districts, appointing Maximian co-emperor. The Eastern Empire eventually adopted Greek and the Western kept Latin, reflecting cultural differences between the two halves.

In a battle on the Milvian Bridge in 312, Constantine, who ruled Britain and Gaul, defeated his former ally Maxentius, who controlled Rome and North Africa. Constantine believed that the Christian God had helped secure his victory.1 In 313 a.d., Constantine met Licinius I in Milan, where in the Edict of Milan, they removed penalties for professing Christianity and returned confiscated Church property. Christianity had developed an organized global network of churches under the leadership of a Pope after the death of Jesus. Christians were known for their loving relationships, honesty, and self-sacrifice based on the teachings of Jesus and the Church. While many of the emperors had seen Christianity as a threat, Constantine believed this religion could serve the moral needs of the Empire.

Even though there was one elected Pope in Rome, there were many divisions and theological differences among the Christians, particularly Arianism in the East. Constantine sought to create an official unified Christian doctrine and called the Council of Nicaea in 325 c.e. The Council summoned all the Bishops of the Christian Church to this ecumenical (meaning “worldwide”) council. All 1800 bishops (1,000 from the East and 800 from the West) were invited and over 300 are said to have attended. The Council produced a significant statement on official Christian doctrine known as the Nicene Creed, which defined the nature of God in a Holy Trinity: The Father, the Son, and the Holy Spirit. The Council’s resolutions were intended for the whole church, and recitation of the Creed encouraged.

Christianity was favored differently by the following Emperors until Theodosius I. After a series of decrees by Theodosius had increased influence of the Church. In 390, Ambrose, bishop of Milan, who was already persecuting non-Christians, excommunicated Theodosius, who had ordered the massacre of 7,000 inhabitants of Thessalonica in response to the assassination of his military governor there. Theodosius performed several months of public penance to be returned to the graces of the Church. In 391, he declared Christianity as the only legitimate imperial religion, ending state support for the traditional Roman religion.

Theodosius had gained the zeal of a convert, using state power to persecute heretical sects and non-Christian religions, and end the remnants of Greco-Roman civic paganism. The eternal fire in the Temple of Vesta in the Roman Forum was extinguished, the Vestal Virgins were disbanded, the Olympic Games ended, and witchcraft was punished. Theodosius’ penance became a precedent for the superiority of the Church over the temporal rulers in matters of faith.

Defining Roles of Church and State

Which sphere is responsible for what? And, how should leaders of each social sphere, the emperors and the popes, relate to one another? These were primary questions of the Middle Ages. They were answered differently in the Eastern and Western halves of the Roman Empire. In the East, which lasted until the fall of Constantinople to the Ottoman Empire in 1453, the Church remained subservient to the Emperor. It was the West, which fell to the Ostrogoths in 476 c.e., the Church had more independence.

The struggles for power between kings and popes shaped the western world between the fifth century and the beginnings of the Reformation in the early sixteenth century. The relationship between the Church and the Empire went through a number of developments. The breakup of the Empire in the West left several warring states, but all were officially Catholic because Christianity had been the official religion all over the Empire.

Economically, the Church was independent of the Kings. It was a feudal landholder, not only in Rome but throughout the world, with its own source of income from monasteries and farms. It also employed soldiers to defend its properties. This gave the church temporal power to assist or coerce kings and emperors.

The King Over the Pope

The Ostrogoth Kings (493 to 537) wanted a say in who became a Pope but otherwise interfered little with the affairs of the Church. Then in 537 Justinian I, the Eastern Emperor, reacquired Rome. The period from 537 to 752 was called the Byzantine Papacy when popes required the approval of the Byzantine Emperor for episcopal consecration, if not outright selection.

Popes were traditionally elected by bishops and approved by church members, but after Christianity became the official religion of the Empire, emperors claimed the authority to approve popes. Sometimes emperors bribed bishops to get the popes they wanted elected.


When the Empire was divided there were frequently several claimants to the papacy and, in addition to lists of official popes, there were at least 40 other claimants known as anti-popes.2“Top 10 Antipopes,“ Historians debate whether all the popes sitting on St. Peter’s chair in Rome were official popes or some of them antipopes.


With the increased power of the church, there was plenty of corruption. People with power and wealth used money to both influence the selection of popes and the policies of popes. The buying or selling of ecclesiastical privileges was known as simony. For example, popes and priests would sell pardons to wealthy people they had excommunicated. They would sell sacred objects for private gain. They would sell administrative offices.

The Pope Over the King

This influence weakened in the seventh century, and Rome, the center of Latin culture, was more oriented toward the West.

The power of the church rivaled, if not exceeded the power of Emperors in 800 a.d., when Pope Leo III crowned Charlemagne, King of the Franks and Lombards, as Roman Emperor. This both brought the protection of the Western Empire to the Church and significantly elevated the power of the Church in the temporal world. The precedent was set. To become an Emperor in Western, one had to be crowned by a pope.

Gregorian Reform

The Gregorian reforms were stimulated by the papal election decree (1059) of Nicholas II which removed kings from the election of popes. This was part of the Investiture Controversy that served to further separate Church and State.  Pope Gregory VII then reorganized the structure of Church institutions to be independent of secular powers. The primacy of the pope over kings was asserted, and the role of the laity in ecclesiastical affairs was reduced. In short, the Church took on more of the form of a feudal state than a community of believers. The church began to flex its temporal muscles with these reforms.

The Crusades

The most notable impact of Church ascendancy was the Crusades to retake the Holy Land (Jerusalem, including parts of Palestine and Syria). Ironically, Urban II called for the First Crusade (1095) under the banner of the Peace of God. The Peace of God was a movement limit warfare to protect the lives of clergy, travelers, women, and cattle and others unable to defend themselves against brigandage. With the loss of large areas of land by the Byzantine Empire to the Muslims by the 11th century, the Crusades were justified to keep the Holy Lands and to ensure safe passage for pilgrims.

After the Norman Conquest of England in 1066, Western Europeans had the capacity to launch a major military undertaking like the Crusades to aid their fellow Christians in the East. There were many military expeditions to the Holy Land until Christians were finally driven out of Jerusalem in 1291. Crusading was not limited to war for the Holy Land, but to fight Muslim encroachment in Spain, and even enemies in the Baltic region.

Crusades and pogroms were also used against heretical churches within the Empire. Particularly famous was the massacre at Béziers in July 1209. Pope Innocent III declared a crusade to eliminate the heretical Catharism in Southern France. When the town was invaded and it was discovered that both loyal Catholics and the herectics lived in the town, the soldiers are reported to ask the abbot what to do, and the abbot supposedly said, “Kill them all for the Lord knoweth them that are His.” (2 Tim. ii. 19) and so countless number in that town were slain.3Medieval Sourcebook: Caesarius of Heisterbach: Medieval Heresies, Fordham University.

The abuse of official Church teaching was used to incentivize fighters for religious conquests. Many crusaders believed that a “crusade indulgence” officially absolved all their previous sins and ensured that they would not be punished in the afterlife. This doctrine parallels the Muslim doctrine still taught to some jihadists today that they will receive a reward of 72 virgins in the afterlife.

Protestant Reformation

Support for crusading declined rapidly during the 16th century after the Renaissance, the Protestant Reformation, and the rise of humanism. These all challenged papal authority in society. Protestant Reformers wanted to return the Church to the spiritual teachings of Jesus and the Bible.Then, the publication of the Gutenberg Bible and widespread literacy enabled average people to compare the Bible to the teachings of the Church. Humanists, who promoted ideas of just war, did not support offensive war or the harshness of the Crusades.

The Protestant Reformation did not end the idea of official state religion, but the plurality of religions weakened the power of the Catholic Church over all of Europe. Princes and Kings began to choose which religion would be the official religion on their territory. A period of brutal religious wars between states, and pograms within states existed from the time of Luther until the Peace of Westphalia (1648), after the Spanish Empire lost control of the Netherlands. Even though the Dutch Reformed Church was the nominal church of Holland, religious freedom was guaranteed to facilitate the support of members of other religions for the new constitution there. 

The Catholic Church had lost its monopoly on truth in Western culture. Yet, it continued to claim its authority with the doctrine of the infallibility of the teachings of the Pope at the First Vatican Council (1869-1870). It was a hollow claim, however, as religious pluralism was becoming more widely accepted. In founding of the United States, the doctrine of separation of Church and State became official.

[1] “Top 10 Antipopes,“

[2] “Medieval Sourcebook: Caesarius of Heisterbach: Medieval Heresies, Fordham University.