As you know, I, Nichiren, have been eagerly studying since childhood and began praying when I was twelve years old to Bodhisattva Space Repository to help me to become the wisest in Japan. The reason for my prayers was complicated, too complicated to explain here in detail. Later I first began to study the doctrines of the Pure Land and Zen Sects. Then I studied the doctrines of the Tendai and Shingon Sects on Mt. Hiei, at the Onjōji Temple, and on Mt. Kōya. I further studied the doctrines of the various sects at temples in Kyoto and the provinces, but these studies did not serve to clear up the doubts I had in mind about Buddhism.

In my initial prayer I made a vow that: I would not favor any particular sect; I would adopt whichever sect that provided the evidence of being the teaching of the Buddha and was reasonable; I would be guided solely by the sūtras, not by the commentators in India, translators and minister-masters in China; I would not be afraid, regarding the doctrines of Buddhism, of even being punished by a king, not to mention persecutions by the people below him; I would not follow instructions against the Buddha’s teachings even if they were given by my parents, teacher and elder brother; and that I would speak up honestly as expounded in the sūtras regardless of whether or not people believed in me.

Ga Ryōkan-tō Gosho, A Letter Refuting Ryōkan-bō and Others, Writings of Nichiren Shōnin, Biography and Disciples, Volume 5, Page 60

It is important to remember that Nichiren was not an ordinary priest. He made a great vow and even his detractors have to agree that he kept that vow.