If the concept of the Eternal Buddha (enlightened since the infinite past) had not been expounded in the Lotus Sūtra, then all the Buddhas throughout the past, present and future would have been mere momentary Buddhas, due to the principle of impermanence. It would be as futile as having no sun or moon even though there are many stars in the sky, or as plants trying to grow in the ground without soil. This conclusion has been based on your recognition, however, of the supremacy of the Lotus Sūtra, which teaches the concept of the Eternal Buddha and enlightenment of all people of the Two Vehicles (of śrāvaka and pratyekabuddha).

Shōjō Daijō Fumbetsu-shō, The Differences between Hinayāna and Mahāyāna Teachings, Writings of Nichiren Shōnin, Doctrine 2, Page 194