Unlike some cultures where a single religion is  dominant, Korean culture includes a wide variety of religious elements  that have shaped the people's way of thinking and behavior. In the early  stages of history in Korea, religious and political functions were  combined but they later became distinct.
Historically, Koreans  lived under the influences of Shamanism, Buddhism, Taoism or  Confucianism, and in modern times, the Christian faith has made strong  inroads into the country, bringing forth yet another important factor  that may change the spiritual landscape of the people. The rapid pace of  industrialization which occurred within a couple of decades compared to  a couple of centuries in the West, has brought about considerable  anxiety and alienation while disrupting the peace of mind of Koreans,  encouraging their pursuit of solace in religious activities. As a  result, the population of religious believers has expanded markedly with  religious institutions emerging as influential social organizations.
Freedom  of religion is guaranteed by the Constitution in Korea. According to a  2005 social statistics survey, 53.1% of Koreans follow a specific  religious faith. Buddhists account for some 43% of the religious  population, followed by Protestants at 34.5% and Catholics at 20.6%. 
Buddhism
Buddhism is a highly disciplined philosophical religion which  emphasizes personal salvation through rebirth in an endless cycle of  reincarnation.
Buddhism was introduced to Korea in A.D. 372  during the Goguryeo Kingdom period by a monk named Sundo who came from  the Qian Qin Dynasty in China. In 384, monk Malananda brought Buddhism  to Baekje from the Eastern Jin State of China. In Silla, Buddhism was  disseminated by a monk Ado of Goguryeo by the mid-fifth century.  Buddhism seems to have been well supported by the ruling people of the  Three Kingdoms because it was suitable as a spiritual prop for the  governing structure with Buddha, like the king, serving as a venerated  symbol of authority.
Under royal patronage, many temples and  monasteries were constructed and believers grew steadily. By the sixth  century monks and artisans were migrating to Japan with scriptures and  religious artifacts to form the basis of early Buddhist culture there.
By  the time Silla unified the peninsula in 676, it had embraced Buddhism  as the state religion, though the government systems were along  Confucian lines. Royal preference for Buddhism in this period produced a  magnificent flowering of Buddhist arts and temple architecture,  including Bulguksa Temple and other relics in Gyeongju, the capital of  Silla. The state cult of Buddhism began to deteriorate as the nobility  indulged in a luxurious lifestyle. Buddhism then established the Seon  (Zen) sect to concentrate on finding universal truth through a life of  frugality. 
Lotus Lantern Festival - A lantern festival is held to commemorate the birth of Buddha on a weekend before the date (April 8th on the lunar calendar). 
www.llf.or.kr The rulers of the succeeding Goryeo Dynasty were even more  enthusiastic in their support of the religion. During Goryeo, Buddhist  arts and architecture continued to flourish with unreserved support from  the aristocracy. The Tripitaka Koreana was produced during this period.  When Yi Seong-gye, founder of the Joseon Dynasty, staged a rebellion  and had himself proclaimed king in 1392, he tried to remove all  influences of Buddhism from the government and adopted Confucianism as  the guiding principles for state management and moral decorum.  Throughout the five-century reign of Joseon, any effort to revive  Buddhism was met with strong opposition from Confucian scholars and  officials.
When Japan forcibly took over Joseon in 1910, it made  attempts to assimilate Korean Buddhist sects with those of Japan. These  attempts, however, failed and even resulted in a revival of interest in  native Buddhism among Koreans. The past few decades have seen Buddhism  undergo a sort of renaissance involving efforts to adapt to the changes  of modern society. While the majority of monks remain in mountainous  areas, absorbed in self-discipline and meditation, some come down to the  cities to spread their religion. There are a large number of monks  conducting religious research at universities in and outside Korea. Seon  (meditation-oriented Korean Buddhism) has been noticeably growing with  many foreigners following in the footsteps of revered Korean monks  through training at Songgwangsa Temple in Jeollanam-do Province and Seon  centers in Seoul and provincial cities. 
Confucianism
Founded by Confucius in the 6th century B.C., Confucianism is more a  moral code of conduct than a religious belief. It is a system of ethical  precepts - benevolent love, righteousness, decorum and wise leadership -  designed to inspire and preserve the proper management of family and  society. Still, Confucianism can be seen as a religion without a god  because as the ages have passed, some followers have canonized the  founding sage and religiously follow the principal disciplines of his  system.
Confucianism was introduced along with the earliest  specimens of Chinese written materials around the beginning of the  Christian era. The Three Kingdoms of Goguryeo, Baekje and Silla all left  records that indicate the early existence of Confucian influence. In  Goguryeo, a state university called Taehak was established in 372 and  private Confucian academies were founded in its provinces. Baekje set up  such institutions even earlier.
Unified Silla sent delegations  of scholars to Tang China to observe the workings of the Confucian  institutions firsthand and to bring back voluminous writings on the  subjects. For the Goryeo Dynasty in the 10th century, Buddhism was the  state religion, and Confucianism formed the philosophical and structural  backbone of the state. The civil service examination of Gwageo, adopted  after the Chinese system in the late 10th century, greatly encouraged  studies in the Confucian classics and deeply implanted Confucian values  in Korean minds.
The Joseon Dynasty, which was established in  1392, accepted Confucianism as the official ideology and developed a  Confucian system of education, ceremony and civil administration. When  Western powers and Japan began using military incursions in the late  19th century to pressure Korea to open up, the Confucianists raised  "righteous armies" to fight against the aggressors.
Efforts were  also made to reform Confucianism and adapt it to the changing conditions  of the times. These reformists accepted the new Western civilization  and endeavored to establish a modern, independent government. Also,  during Japan's colonial rule of Korea, Confucian reformists joined many  independence movements to fight against imperial Japan. Today, Confucian  ancestral worship is still prevalent and filial piety is highly revered  as a virtue in Korean society. 
Catholicism
The tide of Christian mission activities reached Korea in the 17th  century when copies of Catholic missionary Matteo Ricci's works in  Chinese were brought back from Beijing by the annual tributary mission  to the Chinese emperor. Along with religious doctrine, these books  included aspects of Western learning such as the solar calendar and  other matters that attracted the attention of the Joseon scholars of  Silhak, or the School of Practical Learning.
By the 18th century,  there were several converts among these scholars and their families. No  priests entered Korea until Chinese priest Zhou Wenmo visited Korea in  1794. The number of converts continued to increase, although the  propagation of foreign religions on Korean soil was still technically  against the law and there were sporadic persecutions. By the year 1865,  two years after the xenophobic prince regent Daewongun came to power, a  dozen priests presided over a community of some 23,000 believers. 
In 1925, 79 Koreans who had been martyred during the  Joseon Dynasty persecutions were beatified at St. Peter's Basilica in  Rome, and in 1968 an additional 24 were honored in the same way.
During  and after the Korean War (1950-53), the number of Catholic relief  organizations and missionaries increased. The Korean Catholic Church  grew quickly and its hierarchy was established in 1962. The Roman  Catholic Church in Korea celebrated its bicentennial with a visit to  Seoul by Pope John Paul II and the canonization of 93 Korean and 10  French missionary martyrs in 1984.  
It was the first time that a canonization ceremony was held outside  the Vatican. This gave Korea the fourth-largest number of Catholic  saints in the world.
Protestantism
In 1884, Horace N. Allen, an American medical doctor and Presbyterian  missionary, arrived in Korea. Horace G. Underwood of the same  denomination and Methodist Episcopal missionary, Henry G. Appenzeller,  came from the United States the next year. They were followed by  missionaries from other Protestant denominations. The foreign  missionaries contributed to Korean society by rendering medical services  and education as a means of disseminating their beliefs. Korean  Protestants like Dr. Seo Jae-pil, Yi Sang-jae and Yun Chi-ho, all  independence leaders, committed themselves to political causes.
The  Protestant private schools, such as Yonhi and Ewha, functioned to  enhance nationalist thought among the public. The Seoul Young Men's  Christian Association (YMCA) was founded in 1903 along with other such  Christian organizations. The organizations carried out socio-political  programs actively, encouraging the inauguration of similar groupings of  young Koreans. These groups pursued not only political and educational  causes but also awakened social consciousness against superstitious  practices and bad habits, while promoting the equality of men and women,  elimination of the concubine system, and simplification of ceremonial  observances. 
Indigenous Religions
The fall of the Joseon Dynasty and the coming of the Japanese occupation spurred the formation of several new faiths.
Won-Buddhism  was founded to lead all sentient beings drowning in the sea of  suffering to an immeasurable paradise. It is a faith based on moral  training and fortitude and the quest for truth. The name Won-Buddhism,  Wonbulgyo in Korean, is a compound of words signifying truth,  enlightenment and teaching: "Won" means unitary circle and symbolizes  the ultimate truth. "Bul" means to enlighten, and "gyo" means to teach  the truth. Therefore, Won-Buddhism is a religion that calls for truthful  enlightenment and the application of that knowledge in daily life.
Chondogyo  was initiated as a social and technological movement against rampant  competition and foreign encroachment in the 1860s. At that time, it was  called Donghak (Eastern learning) in contrast to "Western learning." The  principle of Chondogyo is Innaecheon which identifies man with  "Haneullim," the God of Chondogyo, even though he is not exactly the  same as God. Every man bears Haneullim in his mind and this serves as  the source of his dignity, while spiritual training makes him one with  the divine.
Daejonggyo, a nationalist religion that worshiped  Dangun, played a critical role in leading the Korean independence  movement during the 1910s and 20s. 
Islam
Although there were trade and diplomatic exchanges between the Goryeo  Dynasty and the Islamic world, these contacts dried up during the  Joseon Dynasty. The first Koreans to be introduced to Islam in more  recent times were laborers dispatched to northeastern China in the early  20th century as part of imperial Japan's colonial policy. A handful of  converts returned home after World War II. They lived alone with their  new faith until the Korean War brought Turkish troops here on the side  of the UN forces. The Turks invited the Korean converts to join them in  prayers. Korean Islam's inaugural service was held in September 1955,  followed by the election of the first Korean imam. The Korean Islamic  Society was expanded and reorganized as the Korean Muslim Federation in  1967, and a central mosque was dedicated in Seoul in 1976.