Śākyamuni Buddha preached the Hinayāna precepts as an introductory step for a while, but He later prohibited it in the Nirvana Sūtra: “If there is a person who says that the Buddha cannot escape extinction, denying the eternal life of the Buddha, the tongue of such a person will surely fall off.”

Later, in the reign of the fiftieth Emperor Kammu, a sage called Saichō (Grand Master Dengyō) appeared. He first studied the Six Schools of Buddhism: Kegon, Sanron, Hossō, Kusha, Jōjitsu, and Ritsu. He further mastered not only the essence of the Zen School but also the schools of T’ien-t’ai Lotus and Shingon Buddhism which had not yet been introduced to Japan. He deeply grasped the comparative profundity and superiority of those schools.

Then, on the nineteenth of the first month of the 21st year of the Enryaku Era (802), Emperor Kammu paid a visit to the Takaodera Temple, where he summoned the fourteen elder masters of the seven great temples of Nara, such as Zengi and Gonsō, and Priest Saichō (Grand Master Dengyō) to discuss the comparative superiority and profundity of the doctrines as well as the possibility of attaining Buddhahood in the Six Schools of Nara and the T’ien-t’ai Lotus School. Each scholar of these schools claimed that his own school was superior in all the teachings of the Buddha, but they were all refuted by Priest Saichō.

Later, Emperor Kammu sent Wake no Hiroyo as his messenger to the seven great temples of Nara in order to reprimand their masters, who submitted a letter of apology written jointly to the emperor. In this letter of apology, the fourteen masters stated: “Henceforth, all the people in this Sahā World, boarding the wonderful and perfect ship of the Lotus Sūtra, crossing the sea of birth and death, will be able to reach the other shore of enlightenment.”

Shimoyama Goshōsoku, The Shimoyama Letter, Writings of Nichiren Shōnin, Volume 5, Page 68-69