South Korea, nor North Korea, is a "Christian republic". In the South, only 53% identify with a religion and the state is secularist. In the North, the percentage is even lower and the state religion is probably the Kim cult. Christianity is not even the largest religion in either Korea, in the South it's still Buddhism. Also, you sound like a crusader when you say that Korea has turned from a backwards state to a "Christian republic", but if you're implying that South Korea is successful because of Christianity then I don't agree. In fact, I think Christianity has only caused a multitude of sinister Korean cults.
South Korea is successful because of the Japanese occupation and post-war support from the United States. The Japanese modernized the country, and the Americans protected the South from Communism and helped secure a pro-Western and pro-capitalist leadership.