When HBS members meet, we say Arigatō gozaimasu ありがとうございます as a greeting. It sounds strange to other people because usual greetings between Japanese are Ohayō おはよう (in the morning), Konnichiwa こんにちは (in the afternoon), and Konbanwa こんばんは (in the evening). We say Arigatō or Arigatō gozaimasu (which is more polite) when we want to thank someone. Arigatō is the word originally derived from Arigatai 有難い, which means that “(something) rarely happens.” Gozaimasu is just a polite word for “there is.” Therefore, Arigatō gozaimasu originally means being grateful for something (e.g. help, kindness, advice, chance, etc.) that is rare.
Nichiren Shōnin stated in his treatise, Shugokokka-ron (守護国家論 – Treatise on Protecting the Nation): “The chances of our being born in the three evil realms [*] are more numerous than particles of dust on earth, while chances of our being born in the human realm are as scarce as the specks of dirt on a fingernail. (…) The chances of our encountering provisional sutras preached in the forty odd years before the Lotus Sutra are more numerous than the particles of dust on earth, while chances of our encountering the Lotus and Nirvana Sutras are as scarce as specks of dirt on a fingernail.” [Translated from Japanese.] He teaches us how rare and wonderful being born as a human and encountering Odaimoku (the Buddha’s true teaching) are.
In Honmon Happon (chapters 15 to 22 in Honmon part of the Lotus Sutra), Primordial Buddha revealed that he had chanted Odaimoku, Namu Myōhō Renge Kyō, joyfully, with an unshaken belief in its power when he attained his Buddhahood for the first time in the remote past. In other words, Buddha taught us that we would attain Buddhahood by the same practice as he had done. We just need to chant and spread the teaching of Odaimoku in the world joyfully.
Therefore, the faith in Odaimoku (HBS faith) is a feeling of joy and gratitude caused by encountering the true teachings of Buddha. Since HBS members understand this feeling, we spontaneously coined our greeting – Arigatō gozaimasu.
*Three evil realms: the worlds of hell, hungry spirits and animals where people are tormented for their evil deeds. These three are the lowest of the ten realms.