How do we know what Nichiren actually taught?

The Nichiren Shoshu/SGI doctrine of Nichiren as True, Eternal, and Original Buddha, is ornate rhetoric and meaningless talk. The moment Nichiren revealed the Three Great Secret Laws, nothing remained hidden in the depths, either in the Lotus Sutra nor in the teachings of Nichiren. Faith or lack of faith in the teachings of Nichiren is the issue. 

Accurately following the teachings is different than originalism or seeking out what the Daishonin had in mind, the interpretation of others, or hearsay. Nichiren was a scriptural Buddhist. What Nichiren actually wrote is that which should be believed and embraced. It is not easy to cull what the Daishonin actually wrote because there are many forgeries in his name. But it is not impossible, as proven by the efforts of various scholars and scholar-priests. By utilizing their methods, we can come to know the truth of the teachings. One method is textual parsimony. What this means is that the writings are weighted. Those actually found in Nichiren's hand are weighted more heavily. Those considered more profound by the majority of priests and scholars, for example, the Five Major Works, are weighted more heavily. Those principles found in the majority of the authenticated works too, are weighted more heavily. The principles in the unauthenticated works are accepted or rejected in light of the Five Major Works and in light of the entire body of authenticated works. Utilizing this method, one can forge strong faith and therefore understanding [because fundamentally, Nichiren's teachings are consistent and not contradictory].