Many Roman provincials were Christian higher clergy. Between the legalization of Christianity by Constantine about 313 and the adoption of Christianity as the legal religion of Rome by the emperor Theodosius I in 380, Christian communities received immense donations of land, labour, and other gifts from emperors and wealthy converts. The Christian clergy, originally a body of community elders and managerial functionaries, gradually acquired sacramental authority and became aligned with the grades of the imperial civil service. Each civitas (community or city), an urban unit and its surrounding district, had its bishop (from the Latin episcopus, “overseer”). Because there had been more Roman civitates in the Italian and provincial European areas, there were more and usually smaller dioceses in these regions than in the distant north and east.

During the 5th and 6th centuries, bishops gradually assumed greater responsibility for supplying the cities and administering their affairs, replacing the local governments that for centuries had underpinned and constituted the local administration of the empire. Two bishops, Ambrose of Milan (339–397) and Gregory I of Rome (pope 590–604), wrote influential guidebooks on episcopal and other clerical duties and responsibilities toward congregations. These works set standards for all later bishops and are still observed in many churches.

Besides the bishops and their subordinates the priests, who tended to the spiritual and material needs of Christians living in the world—the “secular clergy”—there also existed communities of monks and religious women who had fled the world. These communities were independent, although nominally under the control of the local bishop, and they followed diverse rules of life—hence their designation as “regular clergy” (from regula, “rule”). The most influential monastic rule in Latin Christianity after the 8th century was that of Benedict of Nursia (c. 480–c. 547). Benedict’s rule provided for a monastic day of work, prayer, and contemplation, offering psychological balance in the monk’s life. It also elevated the dignity of manual labour in the service of God, long scorned by the elites of antiquity. Benedict’s monastery at Monte Cassino, south of Rome, became one of the greatest centres of Benedictine monasticism.

The origins of monasticism lay in the ascetic practices of Egyptian and Syrian monks, which were transplanted to western Europe through texts such as the 4th-century Latin translation of the Life of Saint Antony (by Patriarch Athanasius of Alexandria) and through widely traveled observers such as the theologian and monk John Cassian (360–435). These Mediterranean-wide influences were among the last examples of the communications network of the older, ecumenical Mediterranean world. Monasticism developed and sustained a powerful ascetic dimension in both Greek and Latin Christianity that increased in importance as monasticism itself came to define the ideal of clerical life in the West.

In the case of Martin (316–397), a former Roman soldier turned wandering holy man, monastic asceticism was combined with the office episcopal, as Martin eventually became bishop of Tours in Gaul. He emphasized the conversion of rural pagans, as well as ministering to the urban and rural elites. In the Iberian Peninsula the work of the monk and bishop Martin of Braga (c. 515–580) was also devoted to the religious instruction of rustics. His work provided an influential model for the later conversion of northern and eastern Europe.

While Greek Christians called their church and religion Orthodox, Latin Christians adopted the term Catholic (from catholicus, “universal”). The term catholic Christianity was originally used to authenticate a normative, orthodox Christian cult (system of religious belief and ritual) on the grounds of its universality and to characterize different beliefs and practices as heterodox on the grounds that they were merely local and did not reflect duration, unanimity, or universality. These three characteristics of Latin orthodoxy were defined by the 5th-century monastic writer Vincent of Lérins (died c. 450) and adopted generally throughout the Latin church.

Devotional movements that differed from the norms of orthodoxy were defined as heterodoxy, or heresy. The earliest of these were several forms of Judaizing Christianity and Gnosticism, a dualist belief in asceticism and spiritual enlightenment. Once Christianity was established throughout the empire, other local movements were also condemned. Donatism, the belief among many North African Christians that Christian leaders who had bowed to pagan imperial persecution before 313 had lost their priestly status and needed to be reordained, was the first major heterodox practice to be considered—and condemned—at an imperial church council (411). Other movements were Arianism, which challenged the divinity of Jesus, and Pelagianism, which denied original sin and emphasized purely human abilities to achieve salvation. Other beliefs, usually those that contradicted increasingly normative doctrines of Trinitarianism (the belief that the Godhead includes three coequal, coeternal, and consubstantial persons) or Christology (the interpretation of the nature of Christ), were also condemned as heresy.

Normative Christianity, which was expressed in imperial legislation, church councils, and the works of influential Christian writers, gradually became the faith of Europe’s new regional rulers. Within that broad, universal ideology, however, many of the new kings and peoples based their claims to legitimacy and a common identity on their own versions of Latin Christianity, as expressed in local law, ritual, saints’ cults, sacred spaces and shrines, and saints’ relics. The cults of saints and their relics served to territorialize devotion, and control over them was a distinctive sign of legitimate power. Although the older empire and the new, nonimperial lands in Europe into which a new culture expanded came to call themselves Christianitas (“Christendom”), they were in practice divided into many self-contained entities that have been called “micro-Christendoms,” each based on the devotional identity of king, clerics, and people.