[I]t is stated in the Lotus Sūtra (chapter four, “Understanding by Faith”) that four great Śrāvakas such as Kāśyapa rejoiced in their understanding of the teaching of the Lotus Sūtra enabling śrāvakas to attain Buddhahood, and reported to the Buddha that they had been given invaluable jewels without asking for them. This represents the attainment of Buddhahood by the śrāvaka realm contained in our minds.

Not only the śrāvaka but also Śākyamuni Buddha is within us. For, we encounter such a statement like this in the second chapter of the Lotus Sūtra: “It was My (Śākyamuni’s) original vow to let all beings become like Myself. My vow has now been fulfilled. I have helped them all enter the way of the Buddha.” Does this not mean, that Śākyamuni Buddha, who has attained Perfect Enlightenment, is our flesh and blood, and all the merits He has accumulated before and after attaining Buddhahood are our bones?

Kanjin Honzon-shō, A Treatise Revealing the Spiritual Contemplation and the Most Verable One, Writings of Nichiren Shōnin, Doctrine 2, Page 146