[That] Ichinen-Sanzen exists only in the Lotus Sutra, and attaining buddhahood can be accomplished only with Ichinen Sanzen, leads to the conclusion that attaining buddhahood is limited to the Lotus Sutra. Nichiren Shōnin stated:

“To tell the truth, however, without the power of the Lotus Sūtra, it is impossible for any living being to attain Buddhahood, break through the six lower realms and be reborn in any of the Pure Lands of Buddhas throughout the universe. You must rely on the power of the Lotus Sūtra in order to leave this Sahā World to enter a Pure World. … Accordingly all living beings including bodhisattvas, men of the Two Vehicles, human beings, heavenly beings and animals who will be reborn in Pure Lands throughout the universe do so as royal children of the Lotus Sūtra. It is because children of the Lotus Sutra are all able to become Buddhas.” (Shōjō Daijō Fumbetsu-Shō, WNS2, p. 197)