There are, in my opinion, four main groups of religions in Japan. I will go through each below.
Shintoism is what can be considered the indigenous religion of Japan. It is at least 1,300 years old but possibly much older. It describes the power struggles in its early history in the disguise of creation myths of the country. This points to migration from Korea as the possible source of its history but also indication of much an earlier culture. It is generally a religion of animism, where mountains, trees, the sun, machinery, virtually anything has a spirit. It is also a religion which promotes purity and fertility, both for agriculture and sex. Shintoism is seen as one religion but can be thought of as having facets of folk, state and culture. From the 6th century until 19th century Shintoism was synchronised with Buddhism (see below).
Buddhism was introduced into Japan from China in the 6th century. It is a religion which originated in north-eastern India in the 6th century BCE. Buddhism is based on the teachings of the Buddha. Buddha taught it is possible to find happiness by thinking and living correctly. There are different “denominations” with Jodo-based sects being the most predominant and the Zen sects being second most predominant in terms of temple count.
Christianity was introduced into Japan in the 16th century when Francis Xavier arrived with Christian converts. Although it makes up a small percentage of followers (around 1% of the population) it is nonetheless prominent within Japanese culture. Most weddings are “Christian” weddings with rites performed by a “priest” (read: ‘foreigner’). Christianity functions thus as marriage officiate, while Shinto functions as life celebrating, and Buddhism for Funerary. All major Christian denominations are represented in Japan.
Most ‘new religions’ are based upon one of the “traditional” religions – Shintoism, Buddhism and/or Christianity. Some were established after 1868 – when Japan began its modernisation period – but many sprang up after 1945. While some do have real ideological differences to their foundation religion most new religions were created for tax-break purposes.
Below are some “keywords” in each of the religious groupings.
Shintoism – inari, hachiman, susa, Izumo, Ise, fertility, creation myth, purity.
Buddhism – Jodo, Jodo Shin, Shingon, Tendai, Nichiren, Zen, death.
Christianity – Francis Xavier, Jesuit, Roman Catholicism, Protestantism, marriage.
New religions – politics, tax-haven, Neo-Shintoism, Neo-Buddhism, Neo-Christianity.