Nichiren explains how the founders of the Lotus Sutra went against the Lotus Sutra and Nichiren

"...But the founders who first established these sects failed to discern the true meaning of the sutras upon which they based their teachings. They proceeded only in a shallow manner, employing the sutras in a way that fitted with their own ideas. In doing so, they went against the Lotus Sutra, which means that their teachings were not in accord with the true intention of the Buddha. They were unaware of this, however, and as they proceeded to propagate their doctrines, both the rulers of the nation and the common people came to believe in them. In addition, these doctrines spread to other countries, and many years have gone by since they were first propagated. As a result, the scholars of this latter age, unaware that the founders of these sects were in error, look up to those who practice and propagate their teachings as men of wisdom."

From Letter to Ichinosawa Nyudo:

http://nds1.weebly.com/letter-to-ichinosawa-nyudo.html

“Soka Kyoiku-gakkai is unique in that it is a combination of the Buddhism of Nichiren Daishonin and my own philosophy of values. By becoming a Nichiren Shoshu priest I would be unable to teach anything except the doctrine of their faith, therefore I will not enter this sect.” (Showa Tokko Dan Atsu Shi — History of Oppression by the Secret Police in Showa Period, Part IV.)