This book not only taught me a lot of history that I didn't know, but it actually changed how I view religion as a cultural concept.
Everyone familiar with Japan knows that in the 1500s and 1600s, Christian missionaries came to Japan and began converting some of the local populace. After initial toleration, the Shoguns responded with banning Christianity, Christian (and all foreign) trade, and persecuting Japanese Christians. That is familiar history, and that isn't wrong.
However, that is told from the outsider (Christian) perspective. This book is fascinating because it explores how Japanese elites perceived Christian and other spiritual systems.
For example, picking Christianity, this was characterized not as a foreign religion but rather as a Buddhist heresy. This was apparently a trope within Buddhism of sort of "anti-Buddhisms" cropping up - very similar to Buddhism, except evil and wrong. In this lens, Christianity is a religion that also originates from the West, the first missionaries came from India, there are temples and monks with similar dress and lifestyles, there's quite a few elements that were perceived as similar and then as heretical/demonic. It's fascinating reading Japanese accounts of Christianity from the 1600s through the 1800s, because they've apparently understood the Christian message but reinterpreted God as an evil deity of rebellion. This also explains why Japan limited contact with China as well - the book references the idea that even China has been tainted by this world-spanning demonic heresy.
That's the first several chapters, and that itself is fascinating. The second part of the book focuses on the opening up of Japan to the West (and the rest of the world). One of the treaty elements was freedom of religion - but what does that mean? Is that freedom of inner belief, of home rituals, of public worship, of preaching and conversion? Additionally, what is acceptable internally within Japan? What is the correct worship of the gods, and what is superstition or heresy?
The author makes the convincing argument that this is where religion as a concept is invented in Japan, the idea that there are discrete religious systems such as Christianity, Buddhism, Shinto, etc. This is also where Shinto itself is invented - it is distilled from Buddhism (despite many of the prominent kami being Indian deities themselves), a mythology is retconned, and State Shinto and Sect Shinto emerge as different entities. Buddhism, meanwhile, becomes castigated as a backwards and foreign religion in the decades preceding the Meiji Restoration, a sentiment which only intensifies under modern Japan.
The overall conclusion is that there is not just a dualism between secular and religion in these consciously modernizing states such as 19th century Japan, but rather three parts - religion, secular, and superstition. Somewhat ironically, Japan was able to distill what it perceived as the "virtues" of Christian nations by secularizing them and adapting them to its "Science of the Gods"; certain practices, rituals, and beliefs amongst the citizenry were then categorized as either acceptable religions or as deplorable superstitions and heresies.
I strongly recommend this book. It was genuinely fascinating (and darkly humorous) to read various Japanese commenters interpret Christianity as an evil Buddhist heresy, and that alone is worth the read. But the bigger point about the construction of the concept of religion - as something which all people just "have", where all religions are sort of equivalent - was fascinating to read about in this non-Christian context.