Ah, a question I can sort-of answer. I’m on mobile, so I apologize if my formatting is lost.

Christianity in Japan in China is a complex situation, where the bad reputation has developed due to the political strife the religion and its followers caused in these nations.

In China, Christianity was not percieved as such a bad religion until the 19th century, necessarily. However, midway through the 19th century, the Taiping Rebellion, also known as one of the bloodiest civil wars in Chinese history, occured, pitting psuedo-Christian followers of Hong Xiquan against the Qing Dynasty, causing upwards of 20 million individuals to perish. Since this occurance, the Qing Empire, and following Chinese States have demonized Christianity, despite the Taiping Heavenly Kingdom having followed a loose interpretation of it. This culminates with the embracing of state atheism in the People’s Republic of China, where religious freedoms have been curtailed to prevent interference between Church and State.

In Japan, on the other hand, there is not so much a poor reputation of Christianity itself, but the religion was never allowed to grow within its borders during periods of Christianization elsewhere. During the Sengoku period, numerous Christian missionaries arrived in Japan, ranging from Portuguese to Russians, bringing Catholocism, Protestantism, Orthodoxy. This caused 2 things to happen within Japan. Many of the daimyō subjects on the islands persecuted, killed, or imprisoned these missionaries for spreading heresy, and closed their borders to them, embracing Shinto teachings rather than Christianity. This would go hand in hand with the Sakoku Law of the future. Christian revolts began in Japan but were quickly shut down by various rulers across the islands. Christianity was, in fact, outlawed during this period by the majority of rulers in Japan, until much later, which led to an underground culture of Kakure Kirishitan— hidden Christians. On the other hand, the Japanese Orthodox Church formed, from Russian missionaries, int he late 19th century, and Roman Catholocism from missionaries during the Sengoku period gained traction from missionaries such as Francis Xavier of Portugal. Nagasaki itself became a Japanese Christian hotspot due to the Portuguese developments in the area.

Today, in Japan, Christians aren’t persecuted as they are in China, as no real stigma exists between Christianity and Japan as it does with the Taiping rebellions in China. Christian traditions, in fact, grew in Japan, following World War II, as American soldiers occupied the nation and brought Christmas Commercialism, Valentine’s Day celebrations, and other Christian celebrations with them to the islands. This has led to a strengthening of Christian celebrations in Japan, and more acceptance of the faith despite the vast Japanese secularization brought on by modern reformations of the nation.

Now, on to the Korean part of your question.

Initially, Christianity was introduced to Korea through Chinese books from Italy and France, brought back by diplomats, but was shortly after banned. It was reintroduced once again, and banned once again, as it was seen to have to potential to undermine the nation’s government. In fact, in 1866, prior to Japanese occupation, Korea massacred thousands of Christian practicers due to the Joseon dynasty viewing Christianity as evil. However, none of this is important, as Korea was very minimally Christian until the mass reintroduction of the faith by Americans in South Korea following Japanese occupation in 1945. Christianity grew from a small ~2% in the early 1940s to almost being 30% of the population in 1991. Since then, the faith has grown to encompass even more of the population, and the nation is now covered in churches and religious structures. American occupation played a major role in turning Korea into a primarily anti-Christian nation into one with a Christian culture of its own, with its own sects of Christian religions forming as well.