The voice-hearer (śrāvaka) refers to disciples of the Buddha such as Śāriputra and Kāśyapa. They observed the 250 precepts and practiced the supra-worldly meditation. Moreover, they contemplated the truths of suffering, emptiness, impermanence, and self-effacement; completely eliminated the delusions arising from false views and thoughts; and mastered the supernatural power of being in water and fire. Therefore, they were able to make even the King of the Mahābrahman Heaven and Indra their followers.

Hōren-shō, Letter to Hōren, Writings of Nichiren Shōnin, Volume 6, Followers I, Page 46