The sin of slandering the True Dharma, though repented, is 1,000 times as heavy as that of the five rebellious sins, to say nothing of slanderers of the True Dharma who do not repent and thus would never be released from the Avīci Hell. Therefore, it is preached in the Lotus Sūtra, fascicle 2, (“A Parable” chapter): “Upon seeing those who read, recite, copy and uphold this sūtra, if a person should despise, hate, envy or bear grudges against them, … he will fall into the Avīci Hell, be confined there for a whole kalpa, and be born there again when the kalpa ends. He will repeat this cycle innumerable times.”

Ken Hōbō-shō, A Clarificaton of Slandering the True Dharma, Writings of Nichiren Shōnin, Doctrine 3, Pages 114.