In order to reflect his new understanding of the Buddha and Buddhist practice, Nichiren believed that the time had come for the establishment of a new precept platform. Nichiren taught that it was impractical for the ordinary person in the Latter Age of the Dharma to attempt to approach awakening by merely adhering to a code of conduct. People no longer felt capable of living up to these various sets of precepts; many of those who did had come to realize that morality and ethics alone do not bring anyone closer to awakening. Of course, there were also hypocrites who strictly adhered to the letter of the precepts while violating their spirit. In order to remedy this, Nichiren taught that the true spirit of all the various sets of precepts is expressed in the Lotus Sūtra. Therefore, the most important thing is to simply strive to uphold the Lotus Sūtra in order to transcend one’s imperfections and attain awakening. This is the true fulfillment of all the precepts.

The Manual of Nichiren Buddhism explains this as follows:

Nichiren claimed that the kaidan at Hieizan was established for the priests whose duty was to save the people of the semblance age of the Dharma and that a new kaidan should be established for the priests who would save those of the latter age of the Dharma. He also held that not only priests but also laymen should come to the Kaidan of the Essential Teaching and receive the Fundamental Precept of Nichiren Buddhism, that is to chant the Daimoku, which should be practiced by all living beings, priests or not. (Murano 1995, p. 62)

Teaching, Practice and Proof, a writing attributed to Nichiren, refers to the “fundamental precept” of upholding the Lotus Sūtra as the “Diamond Chalice Precept.” The following passage from the Brahmā’s Net Sūtra is a possible source for this precept: “This precept of the diamond chalice is the source of all Buddhas, the source of all bodhisattvas and the seed of the Buddha nature.” Nichiren realized that if the Wonderful Dharma of the Lotus Flower Teaching, Myōhō Renge Kyō, is the enlightenment of the Eternal Śākyamuni Buddha and therefore the seed of buddhahood, then Myōhō Renge Kyō is itself the Diamond Chalice Precept. By chanting Namu Myōhō Renge Kyō, practitioners would be upholding the Diamond Chalice Precept that embraces all other precepts. Nichiren goes on to say in Teaching, Practice and Proof:

Afterwards, [explain that] the core realization of Myōhō Renge Kyō, which is the main gate of the Lotus Sūtra, contains all the merits of the practices and virtues of all the buddhas of the past, present, and future, which manifests as the five characters. How could these five characters not contain the merits of all precepts? Once the practitioner has this comprehensive Wonderful Precept, even if he wants to destroy it, he cannot. This has been called the ‘Diamond Chalice Precept.’ All buddhas of the past, present, and future keep this precept. All the Dharma-bodies, reward-bodies, and accommodative-bodies become the buddhas of no beginning and no end. The Great Master Tiantai wrote: ‘[The Buddha] secretly put this into all the teachings and did not expound it.’ Now when all people, whether wise or foolish, householder or home-leaver, upper or lower class, of the present latter age of the Dharma train themselves in accord with the view of Myōhō Renge Kyō, why should they not obtain buddhahood?

[The twenty-first chapter of the Lotus Sūtra states:] ‘Therefore, the man of wisdom who hears the benefits of these merits and who keeps this sūtra after my extinction will be able to attain the awakening of the Buddha definitely and doubtlessly. The people of the provisional schools who slip away from this decisive teaching of the three Buddhas (Śākyamuni, Many Treasures, and the emanation buddhas of the ten directions) will definitely end up in the Avici Hell. Similarly, if this precept is so excellent, then all the precepts of the previous provisional teaching will have no merit. Without any merit, the daily rules of abstention are useless. (Kyōgyōshō Gosho in the Shōwa teihon p. 1488 as translated by Yumi and Michael McCormick)