Ryakko shugyo (countless kalpas of practice to reach enlightenment in Buddhism) (歴劫修行)
Ryakko shugyo is the ascetic practices which a bodhisattva (one who vows to save all beings before becoming a Buddha) does for a long period of time while being reincarnated over and over for three generations, past, present, and future.
The source of it is Muryogikyo seppohon (The Sutra of innumerable meanings). Ryak means the passage of time, and ko means time. Therefore, ryakko means a lapse of kalpa.
In Buddhism, ascetic practices have to be done to become a Buddha. It is said that a Bodhisattva practice asceticism, while being reincarnated over and over, for Sangi hyakudaiko (countless time (kalpa)): a long period of time, since he experiences a religious awakening until he becomes a Buddha. From Juju (ten dwellings) to Jueko (ten transferences), the ascetic practices have to be done for the first incalculable eon holding services for 75,000 Buddhas, and from Kangiji: the first stage of Jicchi (ten stages), to Enkoji: the seventh stage, the ascetic practices have to be done for the second incalculable eon holding services for 76,000 Buddhas, then from Fudoji: the eighth, to Hounji: the tenth, the ascetic practices have to be done for the third incalculable eon holding services for 77,000 Buddhas, and finally, the ascetic practices have to be done for 100 eons to acquire happiness and a fortune for obtaining 32 marks. Therefore, this practice is called Ryakko ue (written as 歴劫迂廻 or 迂回; "ue" means a detour), and it came to be called Zankyo (gradual teaching) because it is the teaching of becoming a Buddha little by little and by degrees.
But, the development of Mahayana Buddhism gave rise to the idea that it would take an excessive amount of time, almost endless, just to come closer to a Buddha, and that it would be impossible to ever become a Buddha. Ryuju, as he described the practice as 'a time lapse without reaching the height' in 観三相品 of "Madhyamaka-karika (Fundamental Verses on the Middle Way)," came to the idea that it would be almost never-ending. That means, if so, no one would ever find the reason for Buddhist ascetic practices.
In ten merits of "Muryo gikyo" (Sutra of Immeasurable Meanings) which followed this idea, Sokushitsutonjo (to become a Buddha quickly) was preached as follows. "If the people had a chance to listen to the sutra, it would be a huge profit." "The reason is because they would certainly attain Mujo bodai (perfect Buddhist enlightenment) early if they worked hard in their ascetic practices."
"The people who had no chance to listen to it should know that they missed something huge. "
"Even if they spent an immense and limitless, incalculable eon, they would not attain Mujo bodai after all…"
"Sokushitsu" literally means "quick" or "fast", and "ton" is the antonym for "Zan," also meaning "quickly" or "immediately"; "Sokushitsutonjo" means to become a Buddha quickly.
Following that, it also preached, 'Even if Rokuharamitsu (the six practices of charity, morality, patience, effort, meditation, and wisdom) has not been practiced yet, Rokuharamitsu naturally face mankind.'
Tannen of Chinese Tendai sect later preached Sokushitsutonjo, when he mentioned Ryunyo-Jobutsu (the legend of the Dragon King's daughter who transformed herself into a man in the process of becoming a Buddha, because women could not become a Buddha) in Daibadatta-bon (Chapter on "Devadatta," the 12th chapter of Hoke-kyo (the Lotus Sutra)) of "Hoke-kyo Sutra," by connecting it to Sokushin-Jobutsu (attaining Buddhahood while still in the flesh) in 諸仏行斉無差別品 of "Bosatsu Shotaikyo (The Bodhisattva Womb Sutra)".
The Tendai sect made Hoke-kyo Sutra its basis defining it as the best, and perfected Kyoso Hanjaku (evaluation of sutras) in which they defined the teachings before Hoke-kyo Sutra as no more than the ones coming from ryakko shugyo. Based on this theory, Saicho also deemed that other Buddhist scriptures were inferior to Hoke-kyo Sutra and criticized Nanto rokushu (the six sects of Buddhism which flourished in ancient Nara).
Nichiren also subscribed to this theory and described in "Kanjin Honzon Sho" (Spiritual Contemplation and the Most Venerable One), connecting it with mappo mukai (a concept to deny the validity of precepts in the Final Dharma Age), 'Sakyamuni's Ingyokatoku no niho (the practices done by Sakyamuni over a long period of time and the virtues cultivated for them) are all contained in the five words of myo, ho, ren, ge, and kyo (in Myohorenge-kyo (Hoke-kyo), and if we remember (and honor) the teachings of Buddha with these five words, his kudoku (merits) of cause and effect are naturally transferred to us.'
Fujimon school such as Nichiren Sho Sect preaches that people can become a Buddha by remembering (and honoring) the teachings of Buddha by having honzon (principal image of Buddha), one of Sandaihiho (the three great secret dharmas); even if they don't practice ryakko shugyo.