The oldest religious ideas in Korea are shamanism and animism. Adherents believed that the natural world was filled with both helpful and harmful spirits that could be communicated with by special people, shamans. Most shamans were women, and certain dances, chants, and herbal remedies marked their beliefs. Although very few people practice this religion today, most Koreans still use herbal remedies, and shamanistic dances and chants can be seen in traditional performances.
Buddhism made its way into Korea through monks who traveled from central Asia, across China, into the peninsula about 372. The new religion was allowed to blend in with the shamanistic beliefs at the time. The mountains that were believed to be homes to the spirits became sites of Buddhist temples.
Chinese monks brought Mahayana Buddhism with them. Korean Buddhism is a form of this religion, except that they tried to resolve what they saw as internal inconsistencies. This new approach, founded by monk Wonhyo, was called Tong Bulgyeo (Interpenetrated Buddhism).
Buddhism was the predominant religion during the Three Kingdoms period and became the official state religion under Unified Shilla. Having the king's support, many temples were built in subsequent centuries (thousands of them, rebuilt after wars and fires, still exist today). One unique feature of Korean temples is a small chapel on the side of the main hall, dedicated to a mountain spirit. Usually depicted as an old man with a pet tiger, it is a symbol of native shamanistic beliefs and an attempt to appease local mountain spirits on whose land the temple stands.
Buddhism continued its popularity as the state religion throughout the Goryeo period. However, as the Joseon Dynasty came into power on the peninsula, Neo-Confucian ideology overtook the Buddhist faith. Monks fled major towns and found enclaves in hidden mountain temples. Only after warrior monks helped defend against the Japanese Hideyoshi invasion did the government end its persecution against Buddhists. However, the religion remained subdued until the end of the Joseon period, when it gained more strength during the Japanese occupation.
Today many factions of Buddhism exist in Korea since Buddhism is not a centralized religion. Popular are Seon (which became Zen in Japan) Buddhism; Taego, a modern revival of Cheontae; and the more contemporary Won Buddhism. It's still a bit odd in modern society to see a monk in his gray robes talking on a cellphone, but within the different factions some monks are allowed to marry and have worldly goods. About 45% of the Korean population is Buddhist.
Confucianism, although not a religion, has had the greatest influence on Korean culture. It was an important part of government systems starting from the 7th century and became the official system of belief in the 14th century, during the Joseon Dynasty. Its philosophical systems are still part of the undercurrent of Korean society.
Christianity came to the peninsula when Roman Catholic missionaries arrived in 1794 (although Jesuit writings were brought into Korea more than a century prior). Mostly because Catholic converts refused to perform Confucian ancestral rites, the government prohibited Christianity. Some early converts were executed during the early 19th century, but anti-Christian laws were not strictly enforced. By the 1860s, there were thousands of Roman Catholics in the country, which caused the government to start their persecution. Subsequently, thousands of Catholics were killed.
In the 1880s, Protestant missionaries and more Catholic priests came to Korea, converting a large number of the population. During the Japanese occupation, Christians played a significant role in the country's struggle for independence. After World War II, Catholicism grew rapidly, but Protestantism grew faster. Christianity has now become the majority religion in the country.
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