As Rome declined, early Christianity grew to replace it as the unifying, civilizing force of the West. Christianity emerged from the teachings of Jesus, who preached in the context of Jewish religious-national ideologies represented by the Sadducees, Pharisees, Essenes, and Zealots. Drawing from the Hebrew prophetic tradition, Jesus preached universal love; he also rejected what he viewed as excessive adherence to ritual and the Law, calling instead for a moral transformation of the individual. Further, he drew from Messianic thought by proclaiming the imminent arrival of God's kingdom. Viewed as a blasphemer by the Jewish establishment and a subversive by the Roman government, Jesus was crucified. His teachings sustained small but decidedly Jewish sect. His follower, Saint Paul, went on to break those teachings from Judaism by severing them from Mosaic law and joining them with the more popular aspects of Near Eastern mystery religions. By presenting Jesus as both an historical man and a resurrected savior, by offering Christianity as a universal faith, and by answering the desires of Gentiles, Paul transformed the Jewish sect into world religion.

Assisted by the infrastructure and universalism of the Empire, Christianity spread through its enormous appeal. To people already seeking fulfillment in mystery religions and spiritualized philosophy Christianity offered a meaningful vision of hope and salvation. Its universalism and concern for suffering humanity appealed to disempowered groups-e.g. the poor, women, slaves-and its strong organization offered communal bonds to those alienated by urban life and the impersonal Empire. Its embrace of Greek philosophical language appealed to pagan intellectuals. At first the Roman government tolerated Christianity. Later, however, it began to suspect Christians of disloyalty, and several emperors organized persecutions, the most severe of which occurred under Diocletian. In 313 A.D. Constantine granted Christianity toleration, and in 392 Theodosius made it the state religion, banning all pagan worship. During these centuries, Christianity gradually Hellenized. Despite resistance from conservative leaders, many Christian thinkers drew upon Greek philosophy-especially Stoicism and Platonism-to create a systematic theology that both preserved the Greek rational tradition and undermined the autonomy of reason.

Christian organization mirrored Roman administration by placing bishops in charge of important urban centers and by recognizing one of those-the pope based in Rome-as the supreme bishop. Many Christians rejected the growing wealth and power of the church, founding monastic communities devoted to ascetic practice. The most important monastic leaders were Saint Basil, who standardized monasticism in the east, and Saint Benedict, whose rule standardized western monasticism. Inconsistencies in Scripture provoked doctrinal disputes, including the Arian controversy settled by the Council of Nicea, and the controversy over Christ's nature settled by the Council of Chalcedon. Christianity gradually accommodated itself to social norms, accepting warfare, slavery, and the subordination of women within a Christian framework. Violently anti-Jewish ideas took root, fed by Christian thinkers grew increasingly frustrated with Jews who would not accept Christianity as the fulfillment of Mosaic law. During the last decades of the Empire, early Christian thought reached its fullest development through the work of the Latin fathers. The most important of these was Augustine, whose City of God articulates a comprehensive vision of earthly and spiritual existence. Augustine also completed the break between Christianity and Greek humanism by subordinating reason to faith.

Christianity offered the most compelling alternative to the dying classical values of the Late Empire. Combining elements of the Hebrew ethical tradition and the Greek rational tradition, Christianity became chief civilizing force after the fall of the western Empire.