“Transcending all differences” means that among the Daishonin’s disciples, there is no opposition or rejection of others. “Becoming as inseparable as fish and the water in which they swim” could be understood as the spirit to treasure one another as irreplaceable individuals and to try to bring out the best in one another. In this way, many in body, one in mind means uniting in spirit and supporting one another.

This guidance is an abstract from 'The World of Nichiren Daishonin Writings' with universal value and application

My mentor, second Soka Gakkai president Josei Toda, cherished this passage from “The Heritage of the Ultimate Law of Life” and lectured on it with profound conviction for the benefit of future generations. He boldly declared that the Soka Gakkai is the harmonious community of practitioners dedicated to kosen-rufu through which the lifeblood of the Daishonin flows. He proclaimed that nothing must be allowed to destroy our organization, which he described as more precious than his own life.
President Toda also said that the Soka Gakkai would be victorious eternally through the unity of many in body, one in mind. He strongly emphasized that the realization of kosen-rufu would only be made possible through our efforts to protect and solidify the Soka Gakkai, which is dedicated to actualizing the Buddha’s will and decree. Unity in faith is the key to achieving kosen-rufu.

This guidance is an abstract from 'The World of Nichiren Daishonin Writings' with universal value and application

The Daishonin indicates that the heritage of the ultimate Law of life and death can only be found where people are chanting Nam-myoho-renge-kyo with the spirit of faith of many in body, one in mind, supporting and encouraging one another without discrimination. The Daishonin further says that daimoku chanted with this spirit is the “true goal of propagation,” and that it is through such faith that the great desire for kosen-rufu can be accomplished.

This guidance is an abstract from 'The World of Nichiren Daishonin Writings' with universal value and application

The mentor–disciple relationship in Buddhism is a life-to-life connection based on the Mystic Law. Generally speaking, it is a relationship between two people that takes place when the mentor, or teacher, has something to communicate to the disciple, or student—whether it be knowledge, a skill, or a craft. In the case of Buddhism, however, it is the Law that the mentor is communicating. The mentor–disciple relationship is necessary to correctly practice the Law and ensure its transmission.
Whether engaging in his own practice or instructing his followers in faith, Nichiren Daishonin always placed great weight on the admonition, “Rely on the Law and not upon persons.” He also said that we should “seek out the votary of the Lotus Sutra and make him our teacher” (cf. WND, 278).
Faith in the Daishonin’s Buddhism is always based on the Law. We practice the Law, and we spread the Law. Achieving the lofty and great objectives of attaining Buddhahood in this lifetime and realizing kosen-rufu require that we unite in spirit and advance with unity of purpose. Toward that end, a teacher who correctly practices the Law is vital. That is why the Daishonin’s successor, Nikko Shonin, strictly stated: “Without following the correct relationship of mentor and disciple, there is no attaining Buddhahood.”

This guidance is an abstract from 'The World of Nichiren Daishonin Writings' with universal value and application