It is stated in the Lotus Sūtra, Chapter 23: “Just as the great King of the Brahma Heaven is the father of all living beings, this sūtra is the wise father of all living beings.” The chapter also states: “This Lotus Sūtra is supreme among all sūtras. He who upholds this sūtra is supreme among all living beings.”
Moreover, Grand Master Dengyō declares in his Outstanding Principles of the Lotus Sūtra: “The reason why the Tendai Lotus School is superior to others is the Lotus Sūtra on which the school is based. This is not boasting and slandering others. I pray that a man of wisdom should find out which sūtra is supreme in establishing a school of Buddhism.”
Shingon Shoshū Imoku, Differences between the Lotus Sect and Other Sects Such as the True Word Sect, Writings of Nichiren Shōnin, Doctrine 2, Page 124