The accent on the world of enlightenment represented by chapter 16 of the Lotus Sutra seems at first to concentrate on the Buddha and on the nature of buddhahood. Yet, the exegesis elaborated within the T’ien-t’ai/Tendai tradition develops a religious view which, in various ways, addresses the position of humanity: a true Buddha cannot exist without human beings (because it is from among humans that a Buddha emerges) and human beings cannot exist without a Buddha (because the Buddha represents the essence of humanity).

Nichiren asserts that the Buddha-world is the only reality and at the same time restores the historical perspective as the only context in which the dimension of the absolute open to human beings is concretized. The Buddha’s enlightenment, that is, “the merits acquired by Śākyamuni through his practice,” is epitomized in the five characters of the title of the Lotus Sutra. Therefore, if someone “receives and keeps” the sutra and obtains access to its meaning through the recitation of the title, they will be endowed with these merits. “The Śākyamuni of subtle enlightenment is our blood and our flesh. The merits of his practice, are they not our bones and marrow?” Nichiren writes. Buddhahood becomes a reality of history, not just in history. Nichiren’s emphasis is not on the absolute per se, but on the relative which has to change to become absolute. A shift occurs from the three worlds of universal time (past-present-future) to the actual historical moment, and this gives a social dimension to Nichiren Buddhism. The endowment with the Buddha-world, however, is the exclusive prerogative of the “practitioner of the Lotus”: “One who keeps the sutra is endowed with the Buddha-bodies and performs Buddha’s acts.” The emphasis on a concrete realization of original time leads to the interpretation of the truth represented by the discourse of the Lotus Sutra as a truth which does not exist beyond the confines of history.
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 235

For Nichiren, the revelation of the original Buddha which takes place in the sixteenth chapter has two meanings: that of a theory of the nature of the Buddha, and that of a speculation on the situation of humankind. Śākyamuni’s “original causes and original results” become the turning point which allows postulating the contemporaneity of human beings to the Buddha. Human beings, in their spatial and temporal limitations, share the temporality of Śākyamuni in this world. Nichiren uses a theory elaborated by Chih-i according to which the past of Śākyamuni is reflected in the past of his disciples. This is the tie established, in the original time, between Śākyamuni and those who listen to the sutra or are willing to accept it. From here Nichiren draws the certainty of buddhahood for human beings:

We living beings of this land are since as many kalpas ago as five hundred particles of dust Śākyamuni’s beloved children. [The relation] between a Buddha with ties and the living beings [bound] by karmic ties can be compared to [the reflection of] the moon in the sky floating on clear water. A Buddha without ties in relation to sentient beings is like a deaf man listening for the sound of thunder or a blind man turning to sun and moon.

According to Nichiren, in the second section of the Lotus Sutra Śākyamuni speaks of this Sahā world as the original land, a pure Buddha realm compared to which the other lands of the ten directions are mere conventional worlds. In Chih-i’s exegesis, the “original land” is the land in which the original Buddha attained enlightenment, therefore the realm of only one type of Buddha. This “Sahā world of the original time” contrasts with the Sahā world where human beings live, which retains the characteristics of a “trace-land.” For Nichiren, on the contrary, there is only one Sahā world. Vulture Peak, the place where the Lotus Sutra is taught, represents both this world of ours and the most perfect world, the only possible “paradise.” There is no other reality, neither for humanity, nor for the Buddha. Whereas Chih-i apparently believed in the Western paradise of Amitābha and hoped to reach it after his death, Nichiren considered the assembly on Vulture Peak a symbol of those who, having received the teachings of the Lotus Sutra, are able to transform our Sahā world into a “resplendent land.” (See this blog post.)

In Nichiren’s hermeneutics the original land thus equals the human world. Since the world where humans live is also the original world in which the Buddha attained buddhahood, phenomenal reality becomes the ground of the most complete enlightenment, which opens to ultimate reality. This enlightenment of the Buddha in the remote past justifies the buddhahood of all beings of this world: Nichiren insists that the śrāvakas and pratyekabuddhas who are promised enlightenment in the first section of the Lotus Sutra could never in fact attain it if the original enlightenment of the Buddha described in chapter 16 had not occurred.
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 232-233

Nichiren classifies all the Buddhas of sutras other than the Lotus and the Śākyamuni described in the first section and in the last six chapters of the second section of the Lotus scripture as temporal bodies, or “Buddhas of the Hinayāna.” Only the Śākyamuni who reveals his enlightenment in the past embodies the true Mahāyāna Buddha. To indicate the infiniteness of this Buddha, Nichiren uses the expression “without beginning and without end,” which properly belonged to a context related to Mahāvairocana Buddha and signified an existence not subject to temporal limitations. This expression suggests that Nichiren attributes an eternal nature to Śākyamuni, and at first seems to imply that he envisages a dharmakāya as the only ground of any reality. But Nichiren develops this infiniteness in a different direction.

Nichiren emphasizes that the Lotus Sutra is the only scripture where not only the dharma body, but also the recompense body, and the transformation body are presented as “infinite”: “When other Mahayana sutras speak of ‘without beginning and without end,’ they refer to the dharmakāya only, not to the three bodies.” Nichiren does not regard the distant past represented by the five hundred kalpas as a metaphorical image, but as a concrete reality identifying an active original body, a “Buddha who in the far distant past has truly manifested himself, has truly practiced, and has truly actualized his enlightenment.” Consequently, the meaning that Nichiren attributes to Śākyamuni is not symbolized either by a transcendental body whose existence is set in a world other than ours or by the recompense body of which Chih-i spoke. This “without beginning without end” of the temporal body is most difficult to believe, Nichiren repeatedly suggests, but the infiniteness of the nirmāṇakāya is the crucial evidence that the Buddha has always abided in this world and that his soteriological activity has been constant since the original time.

Thus Nichiren resolves the conflict between the mundane and the ultimate by creating an all-encompassing Śākyamuni Buddha, who maintains characteristics of the historical Śākyamuni (the activity of preaching) and at the same time is endowed with attributes of the dharmakāya (infinite existence). In this way, the dharma world itself comes to be conceived as the phenomenal reality which actualizes the ultimate truth. Borrowing from Tendai terminology, Nichiren calls this reality “a concretely accomplished ‘three thousand worlds in one single thought.’ ”
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 231-232

In chapter [11] of the sutra, Śākyamuni’s emanations materialize, having been asked to gather from the ten directions. Chih-i had already suggested that the emanation-bodies of Śākyamuni prove that he had not attained enlightenment only forty years before preaching the Lotus Sutra, otherwise there could not have been so many kalpa-old beings who had received instruction from him. However, Chih-i did not invest Śākyamuni’s emanations with a universal significance, probably because he did not regard Śākyamuni as the only true Buddha of the universe. Nichiren’s declaration that all Buddhas enlightened in the past are emanations of Śākyamuni is of a different nature: it challenges the equality of all Buddhas and, furthermore, operates as a reduction which unifies all Buddhas, not only those appearing in the Lotus Sutra, but also those appearing in other scriptures of the Buddhist canon. It should be noted that this “absolutization” of Śākyamuni, although reminiscent of the idea that “all Buddhas are just one single Buddha” developed by esoteric Tendai in Japan, does not proceed by equating Śākyamuni with another Buddha already defined as universal, like Vairocana, but rather by including all Buddhas (Vairocana, too) in the person of Śākyamuni.

Nichiren discusses at length how all Buddhas are enlightened because of their relation to Śākyamuni Buddha.

If we consider the stage of results, the many Tathāgatas are Buddhas of a past ten kalpas, one hundred kalpas or a thousand kalpas long. Lord Śākyamuni is a Buddha who has [attained] the complete result of subtle awakening as many kalpas ago as five hundred particles of dust. The various Buddhas of the ten directions such as the Tathāgata Vairocana, the Tathāgata Amitābhā and the Tathāgata Bhaisajyaguru are followers of our original teacher, the Lord Śākyamuni. One moon in the sky floats in the water as ten-thousand [moons]. … This Buddha Abundant Treasures, too, is a follower of the Lord Śākyamuni of the chapter “The Long Life of the Tathāgata.