In the political vacuum that arose in the West because of the invasion by the German tribes, the Roman church was the single institution that preserved in its episcopal dioceses the Roman provincial arrangement. In its administration of justice the church largely depended upon the old imperial law and—in a period of legal and administrative chaos—was viewed as the only guarantor of order. The Roman popes, most notably St. Gregory I the Great (reigned 590–604), assumed many of the duties of the decadent imperial bureaucracy. Gregory negotiated with the Lombard kings of Italy, oversaw public welfare, and was the soldiers’ paymaster. His administrative skill helped lay the foundation for the Papal States, which emerged in the 8th century. Supporting papal claims and responsibilities was the so-called Petrine theory—the idea that the pope was the representative of Christ and the successor of St. Peter.

Although he considered himself part of a Christian commonwealth headed by the emperor in Constantinople, Gregory sought to improve the religious life of the peoples of the West. Under him the church in Spain, Gaul, and northern Italy was strengthened, and England was converted to Roman Christianity. Later popes forged an alliance with the rulers of the Frankish (Germanic) kingdom in the 8th century and succeeded in winning them as protectors of the Papal States when the Byzantine emperor was no longer able to protect Rome. The relationship created a new area of tension, as religious and secular leaders sought to define the exact nature of the relationship between them. From at least the time of Pope Gelasius I (reigned 492–496), two powers, or swords, were recognized as having been established by God to rule. Carolingian rulers maintained that, as holders of one of the swords, they had special rights and duties to protect the church. Indeed, the emperor Charlemagne claimed for himself the right to appoint the bishops of his empire, who were thus increasingly involved in political affairs.

Emperors in the 10th century, building on Carolingian precedent, continued to involve themselves in church affairs. As a result, bishops in the empire were sometimes also the reigning princes of their dioceses, and they were occasionally guilty of being more interested in the political than in the spiritual affairs of their dominions.

These conflicting perspectives were the cause of a series of struggles between popes and secular rulers that began in the 11th century, when lay and religious leaders sought to reform society and the church. Already in the 10th century, monastic reform movements centred at Cluny, Gorze, and elsewhere had attempted to improve the religious life of the monks and establish a new understanding of ecclesiastical liberty. In the 11th century, reformers such as St. Peter Damian and Humbert of Silva Candida provided new definitions of the sins of clerical marriage and simony. These intellectual developments, along with new decrees governing papal elections, led to the virtual elimination of secular interference in episcopal and papal succession. The staunchest supporter of these reforms, Pope Gregory VII, ultimately banned the practice of the lay investiture of bishops and challenged the traditions of sacral kingship. Gregory’s assertion of papal authority, however, was opposed by the German ruler Henry IV. Their conflict eventually burst into the great Investiture Controversy, which became a struggle for supremacy between the church and the monarchy. The resolution of the controversy left the emperor in a weakened state and increased the influence of the secular and ecclesiastical princes.

Although the empire was reconstituted in the 12th century on the basis of Roman law and the understanding of the empire as a distinct sacred institution (sacrum imperium), it broke down during the 13th century as the result of a new struggle between the emperors and several successive popes. The church, however, faced a new challenge in the rise of the European nation-states. Papal ideology had been shaped by the struggle with the emperors and thus was not suited to deal effectively with kings of nation-states. This first became clearly evident in the conflict between Pope Boniface VIII and King Philip IV of France over matters of ecclesiastical independence and royal authority. In 1296 Boniface issued a bull denying the king’s right to tax the clergy, which he withdrew because Philip forbade the clergy to send money to Rome and the pope needed the revenue. In 1301, Philip violated long-standing tradition by trying the bishop of Pamiers in a royal court. Boniface responded in 1302 with the bull Unam Sanctam (“One Holy Church”), the most extreme assertion by any pope of the supremacy of spiritual over secular authority. Revealing how much had changed since the time of Gregory VII, Philip rallied public opinion against the pope, calling the Estates General to session to accuse Boniface of heresy, witchcraft, sodomy, and other crimes. Philip’s adviser, Guillaume de Nogaret, seized Boniface at Anagni, a town near Rome. Although the pope was rescued by local inhabitants, he died from the shock of the capture, and Philip emerged triumphant. Papal fortunes declined even further during the subsequent Babylonian Captivity of the church, when the papacy resided in Avignon (1309–77) and was perceived as being dominated by the French monarchy.

Secular control of the church increased during the Great Schism (1378–1417), and in some parts of Europe it continued even after the schism ended. The schism was partly the result of growing demands for the papacy’s return to Rome. Pope Urban VI settled in Rome and alienated a number of cardinals, who returned to Avignon and elected a rival pope, Clement VII. Popes and antipopes reigning simultaneously excommunicated each other, thus demeaning the papacy. The schism spread great uncertainty throughout Europe about the validity of the consecration of bishops and the sacraments as administered by the priests they ordained. It was perpetuated in part by European politics, as rival rulers supported either the pope in Rome or the pope in Avignon to assert ever greater authority over the church in their realms. The schism contributed to the rise of the 15th-century conciliar movement, which posited the supreme authority of ecumenical councils in the church.

Although the relationship between the temporal and spiritual powers continued to be a matter of concern in the 16th and 17th centuries, the changes brought by the Reformation and the growth of state power recast the nature of the debate. Under King Henry VIII of England a revolutionary dissociation of the English church from papal supremacy took place. In the German territories the reigning princes became, in effect, the legal guardians of the Protestant churches—a movement already in the process of consolidation in the late Middle Ages. The development in the Catholic nation-states, such as Spain, Portugal, and France, occurred in a similar way.

The ideas of the freedom and equality of Christians and their representation in a communion of saints by virtue of voluntary membership had been disseminated in various medieval sects such as the Cathari, Waldenses, Hussites, and the Bohemian Brethren and were reinforced during the Reformation by groups such as the Hutterites, Mennonites, and Schwenckfelders. These groups also renounced involvement with the state in certain respects, such as through military service and the holding of state offices; some of these groups attempted to structure their own form of common life in Christian, communist communities. Many of their political ideas—at first bloodily suppressed by the Reformation and Counter-Reformation states and churches—were later prominent in the Dutch wars of independence and in the English Revolution.

In the Thirty Years’ War (1618–48) confessional antitheses were intermingled with politics, and the credibility of the feuding ecclesiastical parties was thereby called into question. Subsequently, from the 17th century on, the tendency toward a new, natural-law conception of the relationship between state and church began to develop. Henceforth, in the Protestant countries, state sovereignty was increasingly emphasized vis-à-vis the churches. The state established the right to regulate educational and marriage concerns as well as all administrative affairs of the church. A similar development also occurred in Roman Catholic areas. In the second half of the 18th century Febronianism demanded a replacement of papal centralism with a national church episcopal system; in Austria a state-church concept was established under Josephinism (after Joseph II [reigned 1765–90]) through the dismantling of numerous ecclesiastical privileges. The Eastern Orthodox Church also was drawn into this development under Peter the Great.