Nichiren explains the Mutual Possession of the Ten Worlds.

"Answer: In Chapter II, the Buddha declares, 'The Buddhas appear in this world in order to open the eyes of all beings to their Buddha-wisdom.' It means that all beings are endowed with Buddha-wisdom, for otherwise the Buddhas could never open their eyes to it. In other words, the nine realms below the Buddha-land contain the Buddha-land. In Chapter XVI the Buddha says: 'A long time has passed since I attained Buddhahood. The duration of my life is, indeed, immeasurable, countless, everlasting, interminable. Good people, apart from my eternity, the duration of my life secured by fulfillment of the Bodhisattva conducts has not yet expired, but will last twice as long as the above-mentioned time.' Here we see that Bodhisattvahood is part of Buddhahood in that the Buddha was once a Bodhisattva. The fact that Bodhisattvas are found among the beings below the buddhas convinces us that the nine realms including that of Bodhisattva are contained in the Buddha-land." -- The True Object of Worship