The concepts of attaining enlightenment by the Two Vehicles and the Eternal Buddha are vital to the Lotus Sūtra. Although these concepts can scarcely be found in other sūtras, it is not at all uncommon in the Lotus Sūtra. The doctrine of Ichinen Sanzen, or the “Three Thousand Existences in One Thought,” is the rarest and the most miraculous. This theory is revealed neither in the Flower Garland nor Great Sun Buddha Sūtras. Moreover, the founders of the eight Buddhist sects, except for Grand Master T’ien-t’ai, did not know even its name. The great philosophers Nāgārjuna and Vasubandhu of India knew it, but locked this important dharma jewel of Ichinen Sanzen deep within their hearts instead of writing it down for others to see.

Shōjō Daijō Fumbetsu-shō, The Differences between Hinayāna and Mahāyāna Teachings, Writings of Nichiren Shōnin, Doctrine 2, Page 193