You should know also that if those who read the Lotus Sūtra think that the attainment of Buddhahood can also be realized by reading other sūtras, it cannot be said that they have truly read the Lotus Sūtra even though they may seem to believe in it.

Hōon-jō, Essay on Gratitude, Writings of Nichiren Shōnin, Doctrine 3, Pages 35.