Zhiyi had taught that the Lotus Sūtra has the function of “opening and integrating” (J. kaie) the three vehicles within the one vehicle. … Nichiren understood this as opening the nine realms to reveal the buddha realm. But what did it mean in terms of practice? Nichiren’s contemporaries often freely combined copying and reciting the Lotus Sūtra with nenbutsu chanting, esoteric rituals, and other modes of Buddhist devotion. For many Tendai scholars of the day, the distinction between true and provisional teachings did not mean renouncing practices other than the Lotus Sūtra. It would indeed be a mistake, they said, to recite other sūtras or chant the names of the various buddhas and bodhisattvas thinking that these represented separate truths. But the one vehicle of the Lotus Sūtra integrates all other teachings within itself, just as the great ocean gathers all rivers. Therefore, they claimed, any practice — whether esoteric ritual performance, sūtra copying, or nenbutsu recitation — in effect becomes the practice of the Lotus Sūtra when carried out with this understanding. Others, however, disagreed, and none more vocally than Nichiren. To argue his point, he inverted the “rivers and ocean” metaphor. Once integrated into the Lotus Sūtra, he said, the nenbutsu, esoteric rites, and other practices lose their identity as independent practices, just as the many rivers emptying into the ocean assume the same salty flavor and lose their original names. Precisely because provisional teachings are integrated into the all encompassing principle of the one vehicle, they are no longer to be practiced as independent forms. At the same time, however, Nichiren insisted that the daimoku contains all truth and blessings within itself. Because the daimoku is all-encompassing, chanting it would confer all the benefits that the religious practices of his day were thought to produce: this-worldly benefits such as protection and healing, assurance for the afterlife, and buddhahood itself. His aim was not to eradicate the spectrum of religious interpretations, but to undercut their basis in other traditions and assimilate them to the Lotus Sūtra alone.

Buddhist sūtras specify two approaches to teaching the dharma: shōju, or leading others gradually without criticizing their present stance, and shakubuku, or actively rebuking attachment to false views. The choice between them, Nichiren said, should depend on the time and place. In his view, in Japan at the beginning of the Final Dharma age — a time and place where the Lotus Sūtra was being rejected in favor of provisional teachings — the confrontational shakubuku method should take precedence over the more accommodating shōju approach.

In the contemporary world, where the violence and suffering brought about by religious conflict are so starkly evident, Nichiren’s emphasis on the exclusive truth of the Lotus Sūtra and his assertive mode of proselytizing sometimes provoke antipathy, as they fly in the face of ideals of tolerance and religious pluralism. Both traditional temple organizations and long-established lay groups of Nichiren Buddhism tend to be more accommodating and to take a milder approach in spreading their teachings, in keeping with Nichiren’s admonition that the method of propagation should accord with the times. Nichiren, however, lived in a very different world, where his conviction of the Lotus Sūtra’s sole efficacy in the age of the Final Dharma demanded resolute opposition to other Buddhist forms. This stance sharply differentiated him from the Buddhist mainstream of his day. Though it drew hostility, it may well have enabled his fledgling community to survive beyond his lifetime by carving out a unique religious identity.

For Nichiren, the Lotus Sūtra alone fully revealed the inherence of the buddha realm in all nine realms of unenlightened beings: By chanting its title, Namu Myōhō-renge-kyō, which instantiates the wisdom of all buddhas, even the most deluded person, he said, can manifest the buddha realm directly. Nichiren likened this to fire being produced by a stone taken from beneath the depths of water or a lamp illuminating a place that has been dark for millions of years.

Ichinen sanzen is a complex and challenging concept, and in his doctrinal instruction, Nichiren frequently concentrated on one of its key component principles: the mutual inclusion of the ten dharma realms (jikkai gogu). Traditional Buddhist cosmology divides saṃsāra or the realm of rebirth for unenlightened beings into a hierarchy of six paths: hell dwellers, hungry ghosts (preta), animals, demigods (asura), humans, and gods (deva). Above these, Tendai doctrine places four more realms characterized by ascending levels of awakening: the two realms of the śrāvakas and pratyekabuddhas, who cultivate the aims of detachment and cessation of desire set forth in the so-called Hinayāna teachings, aiming for the goal of Nirvāṇa; bodhisattvas, who strive for the liberation of all beings; and fully realized buddhas. In contrast to the buddha realm, which represents enlightenment, the other nine realms represent various levels of delusion, or states not yet fully awakened. Being empty of fixed, independent existence, all ten interpenetrate, meaning that each realm contains all ten within itself. Specifically, this means that the Buddha and all living beings are not separate; the buddha realm does not exist apart from oneself. Nichiren explains this in simple terms: “As to where hell and the Buddha exist: some sūtras say that hell lies beneath the ground, while others say that the Buddha dwells in the west. But close investigation shows that both exist within our five-foot body. For hell is in the heart of a man who despises his father and makes light of his mother, just as flowers and fruit are already present within the lotus seed. What we call ‘buddha’ dwells in our mind, in the same way that stones contain fire and that jewels have value intrinsically.”

Nichiren saw the Lotus Sūtra as all-encompassing, containing the whole of Buddhist truth within itself. All other sūtras reveal but partial aspects of that truth. Or, in Tendai terminology, the Lotus Sūtra is “true,” while all other sūtras are “provisional.” What this meant for Nichiren in practical terms was that the Lotus Sūtra alone enables all persons without exception to become buddhas. Nichiren grounded this claim in the “three thousand realms in a single thought-moment” (ichinen sanzen) a principle first articulated by Zhiyi. … In essence, this principle means that at each moment the smallest phenomenon (“a single thought-moment”) and the entire cosmos (“three thousand realms”) mutually pervade and encompass one another. Where Zhiyi had introduced this idea only briefly, Nichiren developed it as the very foundation of his thought.