Question: Why is your religion superior to theirs?
Answer: Because our religion leads to Buddhahood and theirs leads to hell.
Question: The evangelicals, particularly the Pentacostal Christians and the Islamic fundamentalists say similar things about other religions and philosophies. Why should we believe you?
Answer: They say that one can attain Buddhahood through their faiths?
Question: Well, they say they will go to heaven and all others will go to hell.
Answer: I understood what you were getting at. Nichiren was a scriptural Buddhist who asserted that the Lotus Sutra is the highest most profound teaching. He too taught that gratitude was an important aspect of personal development but he never taught that gratitude leads to material abundance. For example, Nichiren Daishonin states:
"...When the Thus Come One Shakyamuni was engaged in bodhisattva practices for countless kalpas, he gathered all of the good fortune and virtue he had gained thereby, divided it into sixty-four parts, and took on their merit. Of these sixty-four, he reserved only one part for himself."
Shakyamuni Buddha gave up his life of abundance as a prince to seek Supreme and Perfect Enlightenment. Nichiren too never sought a life of abundance. Neither sought a life of personal enrichment for themselves, save for 1/64 part of their good fortune. The other 63/64 parts of their good fortune they bestowed upon others.
Besides, for a practitioner of the Lotus Sutra, gratitude is a given. The question is not having gratitude but to whom and how to repay one's gratitude. Lastly, his teaching on the objects of one's gratitude and the nature of gratitude itself differs from theirs. If you have any question on the objects of Shakyamuni Buddha's and Nichiren Daishonin's gratitude or their nature, I will be happy to answer them for you or you can search the Lotus Sutra and Gosho under "gratitude".