The Catechism in Questions & Answers

593. How did Christianity develop in the time before the Middle Ages?

During the great Migration Period (in the fourth and fifth centuries) Christianity gained in strength in both Europe and Asia.
Monasticism, which first came into being in Egypt in the third century, played a special role in the spread of Christianity. One of the principle duties of the monks was to live a life of poverty in accordance with the example of Christ, and to spread the Christian faith. In the Middle Ages, monks and nuns accomplished outstanding achievements in science, and were also involved in agriculture and social issues. Increasingly, Christianity came to define the lives of the people, as well as the culture, politics, and society of Europe.
In the year 1054, tensions led to a split between the Western Church (Roman Catholic) and the Eastern Church (Orthodox).

Monasticism is a lifestyle in which people endeavour to dedicate their entire lives to their religion in isolation from all things secular. Both men and women (nuns) engaged in this “monastic” lifestyle.