The practice of Christianity in Korea revolves around two of its largest branches, Protestantism and Catholicism, accounting for 8.6 million and 5.3 million members respectively. Roman Catholicism was first introduced during the late Joseon Dynasty period. In 1603, Yi Gwang-jeong, Korean diplomat, returned from Beijing carrying several theological books written by Matteo Ricci, a Jesuit missionary to China. He began disseminating the information in the books and the first seeds of Christianity were sown.
In 1758 King Yeongjo of Joseon officially outlawed Catholicism as an evil practice. Roman Catholicism was again introduced in 1785 by Yi Seung-hun. Korean Christians were subject to persecution and hardship.
Many were martyred, especially during the Catholic Persecution of 1801 and later. The Joseon Dynasty saw the new religion as a subversive influence and persecuted its earliest followers in Korea, culminating in the Catholic Persecution of 1866, in which 8,000 Catholics across the country were killed, including 9 French missionaries. The opening of Korea to the outside world in the following years brought religious toleration for the remaining Catholics and also introduced Protestantism. The first Presbyterian missionary in Korea, Horace Newton Allen, arrived in 1884 and remained in Korea until 1890, by which time he had been joined by many others.
The growth of both was gradual before 1945. In that year, approximately 2% of the population was Christian. Rapid growth ensued: in 1991, 18.4% of the population (8.0 million) was Protestant, and 6.7% (2.5 million) was Catholic. The Catholic Church has increased its membership by 70% in the last ten years. Anglicanism in Korea has also experienced significant growth in the recent decades. Protestantism has been a dynamic force, providing a dynamic standard against which Catholics and Buddhists have been forced to compete. It was the inspiration for numerous sects, such as the Unification Church, founded in 1954 by Sun Myung Moon.
The influence on education has been decisive as Christians started 293 schools and 40 universities including 3 of the top 5 academic institutions. In recent years, Protestantism has seen a decline in South Korea due to scandals involving church leadership and conflict among various sects, as well as negative opinions regarding missionary work by the churches as being too aggressive among the general South Korean public.
Grayson argues that Protestantism has been a dynamic force in Korean life, and had a positive impact on other religions. It made for a dynamic competitor against which Catholics and Buddhists had to compete, as well as the inspiration for numerous smaller sects. They adopted many of the methods pioneered by the Protestants. The influence on higher education in Korea has been decisive as the Christians started 293 schools and 40 universities including 3 of the top 5 academic institution. Sukman argues that since 1945 Protestantism has been widely seen by Koreans and the religion of the middle class, youth, intellectuals, urbanites, and modernizers. It has been a powerful force supporting South Korea's pursuit of modernity and emulation of the United States, and opposition to the old Japanese colonialism and Communism of North Korea. Prior to the Korean War (1950–1953), two-thirds of Korean Christians lived in the North, but most later fled to the South. It is not known exactly how many Christians remain in North Korea today, and there is some uncertainty about the exact number in South Korea. It is known that by the end of the 1960s there were around one million Protestants in South Korea, but during the "Conversion Boom" period ending in the 1980s, the number of Protestants increased faster than in any other country. The 2005 South Korean census showed 29.2 percent of the population as Christian, up from 26.3 percent ten years previously. Presbyterian Churches are the biggest Protestant denominations in South Korea, with close to 20,000 churches affiliated with the two largest Presbyterian denominations in the country.
South Korea currently provides the world's second largest number of Christian missionaries, surpassed by the United States. GSM, the missionary body of the "Hapdong" General Assembly of Presbyterian Church of Korea, is the single largest missionary organization in South Korea. South Korean missionaries are especially prevalent in 10/40 Window nations that are hostile to Westerners. In 2000, there were 10,646 Protestant South Korean missionaries in 156 countries, along with an undisclosed number of Catholic missionaries. According to an article published in 2004 "South Korea dispatched more than 12,000 missionaries to over 160 countries in comparison to about 46,000 American and 6,000 British missionaries, according to missionary organizations in South Korea and the West". According to an article published in 2007 "Korea has 16,000 missionaries working overseas, second only to the US". In 1980, South Korea sent 93 missionaries and by 2009 it was around 20,000.