Religion in Japan
The two dominant religions in Japan are Buddhism and Shintoism. Compared to Shintoism, Buddhism has fewer religious rites and ritual practices, and places more emphasis on preaching. Although Buddhism was introduced in 552 and grew in popularity, Japan never saw a full conversion away from Shintoism, the indigenous religion. Despite incredible hurdles that had to be overcome by both sides, Buddhism was gradually incorporated into the culture and mixed with local folk religions.
Although the temples making up the pilgrimage are all Buddhist temples (otera), you will occasionally encounter Shinto shrines (jinja) along the way. The main feature distinguishing between the two is the entrance:
Those entering Shinto shrines must pass through a wooden vermillion gate, called a torii (left/top), while Buddhist temples have a sanmon (right/bottom) as their entrance. This distinction is important, because the Shikoku Pilgrimage is a purely Shingon Buddhist pilgrimage, meaning all temples will be Buddhist temples, including the bangai temples. You will encouter quite a few Shinto shrines on the way, including a few famous ones, and I encourage you to check them out on your spare time.
The 88 Temples of the Shikoku Pilgrimage
The Shikoku Pilgrimage is completed by visiting all 88 temples around the island. The debate as to why these 88 temples were chosen, out of the many more who claimed to be associated with Kūkai and the pilrgimage, continues today. These are the main temples of the pilgrimage that form a circular shape around the island of Shikoku. Some of them are clustered together, others far in between. Some are within bustling cities and small towns, or along the coast, and others require a strenuous hike to reach them on the top of mountains. For this reason, you might find yourself walking for days without visiting one single temple. Out of the 88 temples, 27 are located near the coast or sea level, and 61 reside in the moutains.
Pilgrims may choose to visit Mount Kōya either before or after the pilgrimage if they choose to. In Shikoku, the pilgrimage route begins at Ryozenji, the first temple. Most pilgrims walk clockwise around the island, in numerical order of the temples. This is considered the "normal" way. It is not necessary to visit each and every temple in order along the way. In fact, it is not even necessary to start at Temple 1 and finish at Temple 88, as long as you visit all the temples. For example, I visited Temples 61 to 63 before hiking up to Temple 60. If you start from Temple 1, the pilgrimage then ends at Temple 88, which is also named 結願所 (kechigansho), the place where your vows and wishes are fulfilled. This is not the end of the pilgrimage. After Temple 88, you must return to Temple 1 where you began your journey.
Note that many pilgrims do not complete the full pilgrimage, or at least not all at once. Some may have duties and repsonsibilities to attend to back home, and others may feel more comfortable doing the pilgrimage one portion at a time. A pilgrim I met decided that she would take it one prefecture at a time, and complete the pilgrimage in four different periods of time in life. Many pilgrims also give up during the especially arduous portions of the pilgrims, including the hike between Temple 11 and 12 and the vast areas of wilderness in Kochi Prefecture where temples are far in between.
The vast majority of pilgrims only visit the 88 temples, but you also have the choice to visit more. In addition to the 88 officially recognized temples, there are hundreds of bangai temples around the island. Among these, 20 of them are official bangai (or bekkaku) near the pilgrimage route. You will inevitibly run into some of them along the way, but a few of them require quite a bit more walking to get to. Not everyone visits these bangai; they are completely optional. Information and locations of the 20 bangai can be found in guidebooks and online sources. Just like the main 88 temples, you will still be able to receive a stamp (nōkyō) at each bangai. Some even provide lodging options.
Okunoin (Inner Sanctuaries)
Each of the 88 temples have (or are associated with) an okunoin, inner sanctuaries that house a deity. Mount Kōya has its own okunoin, the mausoleum where Kūkai is believed to be resting. Each Buddhist deity has a legend that can be related to the establishment and mission of each temple, and some bekkaku and bangai are actually the okunoin of one of the 88 temples. At the Kōonji (香園寺) okunoin, for exmaple, Buddhist ascetics use it to perform suigyō, a meditative technique that involves immersing yourself in cold waters under a waterfall, or dousing yourself with buckets of freezing water to achieve a higher level of spiritual awareness.
Each okunoin is detached from the location of its temple. They are small in size (usually the size of a roadside shrine), and reside in either another temple grounds or in the mountains. Visiting okunoin is not a prerequisite to completing the pilgrimage. Although pilgrims seldom visit them, but I would strong encourage you to check out at least one okunoin.
Roadside Shinto Shrines
Occasionally, you will encounter small wooden or stone shrines on the side of trails and roads. These mini shrines reflect the nature-worshipping roots of Shinto, centering on family values and agriculture. Each roadside shrine represents a deity, whose identity is represented by the sculpture inside. A common figure is Jizō, protector of travellers, women in childbirth, and children.