Your challenge is to create a religion that is a combination of Christianity and Shintoism. This is a very interesting scenario since it does not involve just spreading Christianity into Japan but syncretizing it with polytheistic, indigenous Japanese religion. Now you might think "but polytheism is not compatible with Christianity so how could it be combined?". Well there is a precedent or model for the combination of polytheistic gods or spirits and Christianity: Haitian Vodou and Santeria. The basic model for how those religions worked would be that there is some higher, monotheistic entity that is impersonal and not directly accessible but is accessible through his intermediaries, each of which have control over different spheres of nature or reality, and which are worshipped or interacted with instead. This was the way in which African, primarily Yoruban slaves, squared Christianity with their indigenous religion.The cult of Santa Muerte also counts as an example. This sort of model could suit a syncretic Shinto-Christian religion very well with Shinto kami being repurposed as intermediaries like Yoruban gods were.
One way for this to be achieved would be a more successful Shimabara Rebellion which, while likely not going to lead to Christianity being widespread, would at least give it some renown or support as the "peasants' religion" like Buddhism became (to my knowledge) in Japan. Had it become more successful, and had greater support by the local nobility (there were already ronin who had allied themselves with the Christian peasants), you might have had more lords decide to become Christian and thus greater tolerance among the Japanese aristocracy. Similarly, the Shimabara rebels were Catholic and both the base religions of Vodou and Santeria was Catholicism (it appears Catholicism is well-suited for syncretism, who knew).
But you all are more knowledgeable than me. How will you all address the challenge? Bonus points if this religion obtains its own Catholic Church but oriented around this syncretic religion.
One way for this to be achieved would be a more successful Shimabara Rebellion which, while likely not going to lead to Christianity being widespread, would at least give it some renown or support as the "peasants' religion" like Buddhism became (to my knowledge) in Japan. Had it become more successful, and had greater support by the local nobility (there were already ronin who had allied themselves with the Christian peasants), you might have had more lords decide to become Christian and thus greater tolerance among the Japanese aristocracy. Similarly, the Shimabara rebels were Catholic and both the base religions of Vodou and Santeria was Catholicism (it appears Catholicism is well-suited for syncretism, who knew).
But you all are more knowledgeable than me. How will you all address the challenge? Bonus points if this religion obtains its own Catholic Church but oriented around this syncretic religion.