[R]yozan Shimizu, in the Manual of the Nichiren Sect, identifies the Kaidan with the Great Mandala of the Great Śākyamuni. This inconceivable, mystic Kaidan is manifested by the Great Meeting in the Sky, taught in the Hommon part of the Hoke-kyō. The Great Mandala representing it consists of three forms.

1. The Great-Mandala-Secret Platform of Original Existence of Non-Beginning (Mushi honnu no daimandara mitsudan). This Mandala, originally complete and existing originally by itself, cannot be seen; its form is “non-form.” It is the absolute Kaidan.
2. The Great Mandala Secret Platform of the Great Meeting in the Air in the Original Doctrine (Hommon kokū dai-e no daimandara mitsudan.) This manifests the superior and wonderful material forms (ji sō) by opening the “form of non-form” of the first Mandala, whereby all complete meetings are made to exist completely. That is, the whole universe and all beings and things contained in it now become manifested. The non-beginning is made evident by the beginning; the Buddha of original origination reveals himself in “dust-particle-kalpas”; the original sowing of the seed is disclosed by the “delivery” (datsu); and the Great Śākyamuni the Venerable, viz. the Original Śākyamuni reveals himself as Śākyamuni. This Mandala represents the Kaidan of the wide and deep hommon teaching of Hokke which is superior to all other teachings.
3. The Great-Mandala-Secret-Platform adapted to the Period of Mappō (Mappō ōji no daimandara mitsudan.) It is made apparent and understandable to the people of low capacity of the time of the End of the Teaching by the use of written characters, drawn on paper with Indian-ink. The Tathāgata’s great mercy is manifested by this third Kaidan, which is to be propagated by the Honge Bosatsu. …

After the two thousand years of the shōbō and zōbō periods had elapsed, the Great Mandala Secret Platform adapted to the Mappō period was established for the first time. By written characters, drawn on paper with Indian-ink, it gives a visible expression and tangible form to the superior and wonderful Meeting in the Air of the Hommon of Hoke-kyō, and by the “believing mind” of one momentary thought, (ichi nen no shinjin) it introduces the devotee to the Secret Platform of the Great Mandala of Original Existence and Non-Beginning. The believing mind of one moment is therefore the Secret Platform of the Mandala of Mappō and the seed of the complete body of Mahāvairocana, the omnipresent Buddha. This seed, already sown in our mind at the time of non-beginning, possesses completely and naturally all matter and mind, cause and effect, subject and object, all the three thousand dharmas. This Secret Platform of the Great Mandala of Mappō is originally within our mind. However, if the gate of the pagoda of our mind should not be open, we cannot see the Ancient Buddha enthroned within. Therefore the gate must be opened by the key in Śākyamuni’s hand, that is by the Namu Myō Hō Ren Ge Kyō, whereby we throw away all temporal teaching, believing in the Hoke-kyō only.

The reader will have become aware from the above description that the term “Kaidan,” as used by the Nichiren School, has lost entirely its historical meaning, which denotes a platform for the ordination of priests. … It is something entirely mystical which results from the association of the dharmakāya and the nirmāṇakāya with the three forms of kaidan, and particularly from the application of the term “secret” (mitsu) to each of them. The Great Meetings celebrated on these secret platforms are neither comparable with any clerical assemblies at some real ordination, nor are they purely symbolical. The one is expressed by means of written characters; the second is noticeable by intuitive vision only; the third surpasses any representation and observation, being covered by absolute darkness that cannot be penetrated.

The view on the “practical Kaidan” of the Nichiren School was for the first time stated in detail in the Sandai hihōshō [On the three great secret Dharmas]. Therefore, this book has been considered since ancient time as the basis for the Nichirenist doctrine of the Kaidan. There we find stated that at the time when the Law of Kings has merged with the Law of Buddha, a most superior place as excellent as Śākyamuni’s Gṛdhrakūṭa -Pure Land, should be found, and the Kaidan established there.

What have we to understand by this Kaidan as recommended by the Sandai hihōshō? Certainly not a mandala and also no ordination platform in traditional meaning, but evidently a dōjō or place for gaining enlightenment by reciting the holy Title of the Hoke-kyō. This Kaidan seems to have been intended as a central place of initiation and worship for the Nichiren Sect, and as the Holy See of true Buddhism, to be established by imperial grant.

“The Nichiren Sect” in Nanjō [Bunyu]’s Short History of the Twelve Japanese Buddhist Sects contains the following remarks on the Kaidan.

The Kaidan, or “place for receiving instruction in śīla or moral precepts,” of the original doctrine is explained as follows: To keep the śīla is the most important matter of all the divisions of Buddha’s doctrine, whether of the great or the small vehicle of the true or of the temporary. Therefore there is in the original doctrine the first true śīla which is held by Buddha permanently. The Kaidan is the Bodhi-manda (dō jō) or “place for the way,” where the ceremony to receive instruction in the śīla is to be accomplished. The place is now mentioned instead of the law which is to be observed there.

The substance of this śīla is the title of the five characters Myō-hō-ren-ge-kyō. One who believes in this title and observes it, is said to be the holder of the excellent śīla of the original doctrine. The place where he keeps and holds it, is the Pure Land of the Calm Light (Jak-kō-jō-do), i.e. the Kaidan. (pp. 145-146)

The Kaidan of the Nichiren Sect is understood to be a practical Kaidan (ji no kaidan) and not a theoretical Kaidan (ri no kaidan) as found in the Tendai Sect. The former expresses the meaning of Hommon or the Original Teaching, the latter that of Shaku Mon or the Trace Teaching. As stated above, the Kaidan of the Nichiren Sect is the Dōjō or “place for the way” (the term “way” meaning here ”the way leading to enlightenment”) where the believer in the Hoke-kyō recites the Title and meditates on it. Upon attaining enlightenment Śākyamuni once sat in such a place, and every Bodhisattva sits down on such a seat before he becomes Buddha. It is said to be the navel or center of the earth, the terrace of enlightenment raising itself on the bodhi-site, and to be diamond-like; consequently it is also called the “diamond throne.”

Tendai Daishi taught the Three Benefits (san yaku) of planting the seed, bringing it to ripeness, and getting deliverance. The “seed” is the seed of goodness sown into the human heart by the Ancient Buddha of Original Enlightenment, and Tendai Daishi evidently believed that by virtue of this seed all men were able to become Buddhas. Though Nichiren adopted the Three Benefits, he believed that “the seed of goodness” had died. While he agreed that in the Shōbo and Zōbō Period men had the capacity of innate goodness, he maintained that in the Mappō Period men had lost their original goodness. It had to be revived again by the beneficial water of the hommon of Hoke-kyō.