Lately I've been studying some of Nichiren's writings; particularly Rissho Ankoku Ron and some of the earlier essays that lead up to that famous treatise. What has moved into the foreground on this reading is how syncretic Nichiren is in his approach. Perhaps eclectic would be a better word. But what I mean is that Nichiren draws on at least three spiritual traditions and weaves them seamlessly together in his writings.
The first, and most important, is Buddhism. I am continually impressed with the depth and breadth of Nichiren's studies and his ability to make connections and interpretations of all of this disparate material.
The second tradition is Confucianism. It is surprising to me how often Nichiren quotes Confucian works or references well known Confucian stories (well known to his audience) to make a point. He seems to regard Confucian works as proof texts. In addition, many of his interpretations regarding the Lotus Sutra have a distinctly Confucian ring to them; such as Shakyamuni being our parent and teacher. Confucianism often merged the categories of parent and teacher arguing that we should treat our teachers as well as, or even better than, our parents. And, of course, filial piety is the primary virtue of Confucianism.
The third tradition is Shinto. I don't recall Nichiren directly quoting things like the Kojiki, or other sources of Japanese mythos (but I haven't read everything). The Shinto influence is present, though, on many of his mandalas in the form of various Shinto deities. By Nichiren's day most Japanese had merged Shinto and Buddhism into a single entity through a system of correspondences such that the Shinto deities were considered to be manifestations of various Buddhist Bodhisattvas and/or deities. It seems that Nichiren accepted this approach.
Finally, now and then Nichiren quotes from Taoist sources, such as Chuang Tzu. Not very often, and I think the Taoist influence is minor. Confucianism seems more important to him.
Understanding this spectrum of influences has helped me to round out the portrait of Nichiren in my own mind.
Dharmajim