[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.