In Chapter 21, The Supernatural Powers of the Tathāgatas, Sakyamuni says:

Anyone who understands why the Buddhas expound [many] sūtras,
Who knows the position [of this sūtra in the series of sūtras],
And who expounds it after my extinction
According to its true meaning,
Will be able to eliminate the darkness
Of the living beings of the world where he walks about,
Just as the light of the sun and the moon
Eliminates all darkness.
He will be able to cause innumerable Bodhisattvas
To dwell finally in the One Vehicle.

Therefore, the man of wisdom
Who hears the benefits of these merits
And who keeps this sūtra after my extinction,
Will be able to attain
The enlightenment of the Buddha
Definitely and doubtlessly.

These words were especially beloved by Nichiren. Needless to say, for him all the words of the Sutra were compelling. Nevertheless, he considered this chapter, the “Supernatural Powers of the Tathagatas,” to be of singular importance. That is because it is here that Sakyamuni transmits the Four Phrases of the Primary Mystery, the essence of the Sutra, to Superb-Action and the Bodhisattvas from Underground. Nichiren believed that these Four Phrases are the Sacred Title (Odaimoku) of NAMU-MYOHO-RENGE-KYO. He maintained that in this chapter Sakyamuni calls the Bodhisattvas who had welled up from underground, headed by Superb-Action, and transmits to them the Sacred Title, the core of the Sutra manifested in its title. These Bodhisattvas, on their part, will appear in our times, the Age of Degeneration, to disseminate the Sacred Title and save all people, leading them to attain Buddhahood.