From the little I know, the Japanese imperial family is (were ? no idea what the official stance is since the 1946 decree where emperor Hirohito, under American pressure, said he was not an incarnated deity) said to be descendants of Shinto deities. Until last century, being a good citizen, loyal to the emperor, and being a Shinto believer were synonymous.
Even if Korea at first strongly resisted the introduction of Christianity, the religious landscape in Korea had always been less monolithic, and there had been a conflict for some time between Buddhism, which had relocated most of its shrines in the mountains for fear of persecution, and Confucianism, the state ideology of the Joseon dynasty (1392-1910).
Paging @Katsuobushi, who is probably far more knowledgeable about this.