It is stated in the Collection Concerning the Immediate Attainment of Buddhahood by Grand Master Jikaku:

“The Buddha’s wish to teach and save the people is fulfilled in the Lotus Sūtra. Thus, the Buddha emerges in this world and expounds the Lotus Sūtra. While practicing the bodhisattva way before attaining Buddhahood, various Buddhas take four great vows to save the people by causing them to eliminate delusions, master the teaching of the Buddha, and attain Buddhahood. If they complete the practice and are endowed with its resulting merits, they can fulfill the last three vows of the Four Great Vows. However, it is difficult for them to accomplish the first vow to lead the people and enable them to eliminate delusions. Not even the Flower Garland Sūtra can enable the people in the Ten Realms (from the realm of Buddhas to hell) to attain Buddhahood. The same is true with the Āgama sūtras, the Hōdō (various Mahāyāna) sūtras, and the Wisdom Sūtra. Though expounding all the teachings again after the Lotus Sūtra, the Nirvana Sūtra cannot accomplish the Buddha’s will to enable all the people to attain Buddhahood. Now it becomes clear that all living beings of the Ten Realms can attain Buddhahood by the teaching of the Lotus Sūtra. Devadatta who had once fallen into the Hell of Incessant Suffering was assured in the Lotus Sūtra to become Heavenly King Buddha; the daughter of a dragon king and the ten female rākṣasa demons attained Buddhahood; an asura demon was assured of future Buddhahood, and bodhisattvas in the realm of men, heavenly beings and the Two Vehicles and all bodhisattvas who mastered the teachings other than the Lotus Sūtra are all able to enter the perfect realm of the Buddha. It is preached in the “Expedient” chapter of the Lotus Sūtra that the Buddha has accomplished His Vow to lead all the people to the way of the Buddha. Since the realm of men is limitless, there still are people who have not entered the way of the Buddha. However, all the people of the Ten Realms will surely attain Buddhahood solely by the strength of the Lotus Sūtra. The Lotus Sūtra fulfills the will of the Buddha to save all the people.”

Nizen Nijō Bosatsu Fu-sabutsu Ji, Never-Attaining Buddhahood by the Two Vehicles and Bodhisattvas in the Pre-Lotus Sūtras, Writings of Nichiren Shōnin, Doctrine 3, Page 222