Myoho-renge-kyo are the heart of the Lotus Sutra and the eye of all Buddhas

Now, at the beginning of the Latter Day of the Law, I, Nichiren, am the first to set out on propagating, throughout Jambudvipa, the five characters of Myoho-renge-kyo, which are the heart of the Lotus Sutra and the eye of all Buddhas. These five characters are the heart of the Lotus Sutra and the source of the enlightenment of all Buddhas. 16/3

The Writings of Nichiren Daishonin
The Actions of the Votary of the Lotus Sutra
Written to the lay nun Konichi in 1276