Honbutsu (the Primordial Buddha) (本仏)

"Honbutsu" means the Primordial Buddha among numerous Buddha (Nyorai.)

Originally in Buddhism, as may be seen in the legend of Kako-shichibutsu (seven Buddhas in the past), Shakyamuni Buddha was believed to have successfully established such a great religion as Buddhism not within his generation, but as a result of the accumulated good conduct of his many predecessor Buddhas who had attained enlightenment and Buddhahood in the past world.
A group of Buddhists of the Devadatta school regarded as heretical had known that they had not recognized Shakyamuni as Buddha and, instead, worshipped Kengo no San-butsu (the last three Buddhas of one thousand Buddhas who appeared during the period before Shakya-muni's generation.)

In ancient India, after Mahayana Buddhism became popular there, not only Shakyamuni, but also many other persons were respected and adored as Buddha. Those Buddha other than Shakyamuni were called shaku-butsu, which were regarded as temporary figures of Honbutsu. Then, there emerged a belief among the people that the reason why Shakyamuni could become Buddha after his death was not only because of his ascetic practices he made during he was alive, but also his long-lasting practices he had accomplished in his past world. Also Shakyamuni was believed that he had done practices under hosts of many Buddha in his past world. And, Honbutsu was assumed to have been one of those Buddha who gave the most causal influence to Shakyamuni.

The thought of Honbutsu appeared in the Buddhist doctrine of the Tendai sect in the 12th century.
It is said that Nichiren's belief in honbutsu was affected by Chuko Tendai Shiso (literally, thought of half-old Tendai sect doctrine.)
Currently, the Tendai sect is not preaching the thought of Honbutsu. At present, it is the Shoretsu school of Nichiren sect only that strongly insists on the thought of Honbutsu.

Nichiren Honbutsu-ron (the idea to identify Nichiren with Honbutsu):

In the early stage of Buddhism, Shakyamuni was believed to have been enlightened after completion of six-year-practices as a priest. This enlightenment of Shakyamuni was called Shijo-shokaku.
However, somewhat later during the period after Shakyamuni's death, there came to the world a thought that Shakyamuni had already been a Buddha preaching dharma for a long time before he was born into this mortal world, as written in some Buddhist sutra like Hokke-kyo Sutra (the Lotus Sutra), which mentions in its Chapter 16 (the Life Span of the Tathagata), for example, 'it has been immeasurable, boundless hundreds, thousands, ten thousands, millions of nayutas of kalpas since I in fact attained Buddhahood.'
This thought has caused to bring about a belief that Buddhahood may be attained not through practices in this mortal world, but only after completion of mind-boggling ryakko-shugyo (countless kalpas of practices to reach enlightenmment in Buddhism) through the long long circle of transmigration. On the other hand, this thought provoked adverse reactions that no body could ever follow such a long hard practice. Then, there appeared Mappo-shiso (the "end of the world" belief). As written in Hokke-kyo Sutra, Shakyamuni had been Buddha for a definite period which began gohyaku-jintengo (unmeasurable long time) ago. Accordingly, Shakyamuni's teachings will become useless at the age of Mappo (Age of the Final Dharma), the idea of which is called Byakuho Onmotsu (Dharma's decease). At the age of Mappo, people shall be rescued not by Shakyamuni Buddha, but by Nichiren, Konpon-butsu (Primordial Buddha) existing in an infinite time frame which began from infinitely remote past. This idea is called Nichiren Honbutsu-ron.

Nevertheless, Nichiren had never claimed himself as Primordial Buddha. It was some disciples of Nichiren who initiated the idea of identifying Nichiren with the Primordial Buddha, but this idea was doomed to failure as it was considered heretical.

Shaka Honbutsu-ron (an idea to identify Shakyamuni as Primordial Buddha):

This idea is a part of the Buddhist doctrines of Nichiren sect or Hokke sect. It is antagonistic to the above mentioned Nichiren Honbutsu-ron. In the Hokke-kyo Sutra Chapter 16, The Life Span of the Tathagata, Shakyamuni Buddha was described as having an infinite life span, who should be recognized as Honbutsu according to the creed of Hokke-kyo.
This Shakyamuni Buddha, recognized as the Primordial Buddha, was also called Kuon-butsu or Kuon Jitstujo Honbutsu (Primordial Buddha with unmeasurable remote past.)
And, many other unaccountable numbers of Buddha were considered as Shakubutsu (temporary figures of Primordial Buddha.)
Many Buddha appearing in the Sutra of Mahayana Buddhism, including various Buddha existing in all directions as well as in the past, present and future, were all Shakubutsu and no more than copies of Honbutsu or Shakyamuni Buddha. This Honbutsu is not necessarily recognized as identical with Gotama Siddhattha (Shaka), who was born in India of Eurasia continent before Christ and lived in flesh and blood for eighty years, but it may be considered as the spirit of Gotama Siddhattha himself. In this theory, Nichiren is not recognized as Honbutsu, but just a disciple of Buddha.

Nehan-gyo (the Nirvana Sutra), has succeeded the idea of Kuon-jitsujo (Buddha had attained Enlightenment in the eternal past) from Hokke-kyo Sutra, and developed and sublimated it into an idea that the real nature of Budddha shall exist forever even after Nyorai (Buddha) entered nirvana.
This new idea was called "Nyorai Joju Fuheni" (Nyorai is always staying here without any change) or "Sanbo Ittai Joju Fuheni" (A set of three treasures of Buddhism is always staying here without any change) or "Kuon Joju" (Buddha is always staying here forever.)
Nehan-gyo has eventually denied mappo-shiso as a theory of mere expediency for graded preachings. And, everywhere in its sutra, Nehan-gyo preached that it was necessary because wicked Buddhist priests may appear in the future to preach that Nyorai (Buddha) is no longer existing after his death.

[Original Japanese]