Why must we maintain the best ethics and best human nature of mankind?

A society that has sacrificed so much to material wealth that it has forgotten the human heart and the better human aspirations degenerates into something compassionless, doctrinaire, ignorant and ultraconservative. When this happens, fundamental solutions to calamities become impossible. If we protect the truth and are resolute, we are capable of creating peace and prosperity. And the truth we must protect ought to be high and great. Our great truth—the thing that we must protect to the utmost—involves ethics and the best of human nature. But more basic than anything else is our duty to guard the truth of life, the truth that we and the universe are one, and that a single ordinary human thought contains the entirety of universal life.

Buddhism Day by Day
Wisdom for Modern Life by Daisaku Ikeda 16/12

Chanting daimoku with the spirit of many in body but one in mind

All disciples and lay supporters of Nichiren should chant Nam-myoho-renge-kyo with the spirit of many in body but one in mind, transcending all differences among themselves to become as inseparable as fish and the water in which they swim. This spiritual bond is the basis for the universal transmission of the ultimate Law of life and death. Herein lies the true goal of Nichiren's propagation. When you are so united, even the great desire for widespread propagation can be fulfilled. 15/2

The Writings of Nichiren Daishonin
The Heritage of the Ultimate Law of Life
Written to Sairen-bo Nichijo on February 11, 1272