What is remarkable about Catholicism in Korea, is that it was an entirely lay lead phenomena. There was a Jesuit, Father Gregorious de Cespedes who worked in Korea in 1593, to minister to the Japanese living in Korea. But he was not permitted to preach to the Koreans.
Yi Gwang-Jeong, a Korean diplomat traveled from China bring books written by Father Matteo Ricci, the Jesuit missionary to China. Here is where the seed was sown and germinated. Over time these ideas circulated.
It was from these books, not through Catholic missionaries per se, that Christianity came to Korea. Christianity was discussed and passed down through the years THROUGH LAY PEOPLE. No priests and sisters taught it. Strong Catholics grew up and formed communities by themselves. Not until 1836 did French missionaries come.
Persecutions came in 1839, 1846 and 1866. The blood of martyrs watered Korean soil. 1925 saw 79 martyrs beatified. Also 1968 had 24 beatified. Then in 1984, the 103 martyrs were canonized as saints. The Feast Day to celebrate these martyrs is September 20th. N.B. Korea has the 4th largest number of canonized saints of any country in the Catholic world.
O God, Who have been pleased to increase your children everywhere. And Who made the blood of martyrs nourish the seeds of Christianity, grant that we may be defended by the intercession of the Korean martyrs, St. Andrew Kim Tae-gon and companions, and learn from their examples. May we stand strong against persecutions of all kinds. Through our Lord Jesus, your Son, Who lives and reigns with You in the unity of the Holy Spirit, one God, for ever and ever. Amen