The history of extreme discipline and religious practice is, in a way, the human history of Mount Fuji. As Chinese culture was introduced to Japan, from the sixth century on, Chinese philosophy became important. Buddhism, but also Tao and Confucianism, played a big part of this. From around the ninth century, Japanese monks made regular trips to China with the aim of bringing back the authentic Buddhism. They set up mountain retreats where people could escape from the world, connect with the local kami, meditate and seek enlightenment.
Also related to Chinese influence, people believed in the existence of mysterious mountain wizards (sennin,仙人) who lived on the mountains, used magic potions, and became immortal. En no Gyoja was regarded as the perfect mountain wizard, and he is now regarded as the founder of Shugendou (修験道), Japanese mountain asceticism. During the Heian Period, more and more ascetics wandered the mountains of Japan, seeking enlightenment and hoping to attain supernatural abilities. These people did not recognize any big difference between Buddhist deities and the local kami. As the home of powerful kami, Mount Fuji itself became an object of worship.
The ultimate and, to us, utterly incredible goal of extreme mountain asceticism was to become self-mummified and achieve status as a Buddha. Monks endured astonishing hardship and lived on extremely restricted diets. When they finally died of starvation, their bodies did not decompose. At this point, they were believed to have become Buddhas.
Related to this, Matsudai, the Saint of Mount Fuji, was an important figure in the development of the mountain as an object of worship. He was born in 1103, and is the first person known to have definitely climbed the mountain. He went on to climb Mount Fuji hundreds of times. He even established a Buddhist temple on top of Mount Fuji and established permanent religious headquarters at the foot of the mountain. He is believed to have eventually become self-mummified (although there are no remains of this today) and, therefore, to have become a Buddha.
Featured image: Dewa Shrine in Yamagata Prefecture. The Three Mountains of Dewa are a focal point for Shugendou practices.