"Therefore, this one Buddha [Shakyamuni] is none other than all Buddhas, and all Buddhas are thus brought together within the two characters of myōhō."

"Answer: In his dying instructions, the Buddha said, “Rely on the Law and not upon persons.” This means that if what a person says is not in agreement with what is expounded in the sutras, one should not believe it, no matter how fine the person may be. And he also said, “Rely on sutras that are complete and final and not on those that are not complete and final.” Because ignorant and uninformed persons cannot decide for themselves which of the sutras expounded by the Buddha in his lifetime of teaching were preached earlier and which were preached later, or which are shallow and which are profound, they should rely on the sutras that are complete and final." -- On Reciting the Daimoku of the Lotus Sutra

and further down:

"Ordinary people in this latter age cannot depend upon others just because they are said to be wise. The people of our present age can hardly hope to compare to the wise persons of long ago. And thus, even the words of someone who is looked on as ignorant, if they are supported by clear passages of proof in the sutras and treatises, are by no means to be scorned." 

Most importantly, Nichiren writes in this treatise.

"Moreover, the Buddhas figuring in the various sutras do not have the power to confer the benefits bestowed by other Buddhas. To be sure, it is stated that the Buddhas are equal in their enlightenment and that one Buddha is the same as the other Buddhas. And that if one adopts the view that the Dharma bodies of the Buddhas are all equal, then one may say that one Buddha is the same as another Buddha. But in reality, one Buddha does not possess the power to confer the benefits bestowed by all the other Buddhas.

In the case of the Lotus Sutra, however, all the sutras preached in the first forty and more years are included within this one sutra. The Buddhas of the worlds in the ten directions, who are all endowed with the three bodies, are one and all gathered there, for, as it is explained, all are emanations of the one Buddha, Shakyamuni. Therefore, this one Buddha is none other than all Buddhas, and all Buddhas are thus brought together within the two characters of myōhō."