[I]t is said in the fifteenth chapter, “Appearance of Bodhisattvas from Underground,” that Bodhisattva Maitreya begged the Buddha to clear his doubts about His original disciples from the eternal past saying:

“We have never doubted the truthfulness of Your opportune sermons or the words uttered by You and we believe that You know all things thoroughly. We are afraid, however, that those who have just begun to walk the bodhisattva way, if they hear these words of Yours after Your death, might not accept them in faith, causing the sin of destroying the dharma. We beseech You, World Honored One, to explain this to remove our doubts. Good men in the future, too, will have no doubts when they hear it.”

This passage shows that the preaching of the chapter, “The Life Span of the Buddha,” was requested for the sake of the people after the death of Śākyamuni Buddha.

Kanjin Honzon-shō, A Treatise Revealing the Spiritual Contemplation and the Most Verable One, Writings of Nichiren Shōnin, Doctrine 2, Page 155-156