Nichiren on filial piety [gratitude to Shakyamuni Buddha, our father and mother throughout the three existences].

“You should put the Collection on the Choice of the Nembutsu of the Original Vow [Senchaku] aside and cite the sentence from the Second volume of the Lotus Sutra, “But now this threefold world is all my domain, and the living beings in it are all my children” in the beginning as proof and conclude that Shakyamuni is our father. What other Buddha is our father and mother? Even non-Buddhist texts teach that loyalty to the sovereign and filial piety are most important. They also teach that loyalty to the sovereign is derived from filial piety. Therefore non-Buddhist texts is a first step toward the Buddhist scriptures and what they teach about these points are no different from what the Buddhist scriptures teach. They ordain that for all human beings, noble or base, of social rank high or low, nothing is more important than filial piety. Sakkyamuni is our father and mother. All the sacred teachings of the Buddha’s lifetime are the teachings and Sutras that are given to us by our father and mother.” — How To Argue Buddhist Doctrine