As all Mahāyāna sūtras except one do not preach on the basis of the “revealing the truth and merging all the provisional” doctrine, neither the doctrine of obtaining Buddhahood for the Two Vehicles nor that of attainment of Buddhahood by Śākyamuni Buddha in the eternal past is preached in them. The Lotus Sūtra on the other hand, is expounded on the basis of the “revealing the truth and merging all the provisional” doctrine, thus both doctrines of attaining Buddhahood by the Two Vehicles and the attainment of Buddhahood by Śākyamuni Buddha in the eternal past are revealed in it. Accordingly, it would be the sin of slandering the dharma for followers of those Mahāyāna sūtras to criticize the Lotus Sūtra, but criticism of those Mahāyāna sūtras by believers of the Lotus Sūtra does not constitute slander of the dharma.

Ken Hōbō-shō, A Clarificaton of Slandering the True Dharma, Writings of Nichiren Shōnin, Doctrine 3, Pages 133.