Joseon Korea could've actually seen a thriving Christian population if Injo wasn't quite as horrible.
Story goes that after the Manchu defeated the Joseon for the second time (both invasions triggered by Injo's anti-Manchu attitude and the coup that actually got him on the throne), Injo's first two sons had to go to the Manchu as hostages. After the Qing conquered Beijing in 1644, they returned, the eldest, Sohyeon, brought with him Western sciences and Catholicism that Jesuits in Beijing showed him and tried to encourage modernization along western/European lines. Sohyeon was found dead within a year in the King's room, bleeding from the head. Injo had him buried immediately and made the funeral paltry compared to what a crown prince's funeral would've been in those days. He had Sohyeon's children sent to Jeju Island, which is where political dissenters tended to get sent, and his daughter in law executed for treason after she tried to find out what happened to the crown prince.
Anyways, say Injo dies like he should've but earlier so as to reduce the damage he did to the peninsula. Sohyeon apparently had a favourable view towards Christianity, seeing as he brought it with him, and a amiable relationship with the Qing (more open-minded and willing to cooperate with foreign entities, unlike his father). Christianity, being tied to trade with Europeans, could flourish like Islam in SE Asia. Then we have a majorly Christian Korea a few centuries early (current day Korea's about 25% Christian, though the vast majority are undeclared or atheist).