Is there any grammatical flaw in the sentences below?

Our ancestors say[remove s] that we must do good rather than evil, on however humble a scale. Sometimes a seemingly casual act could save someone's life. It ["It" a bit vague here; use "This"] has immense power and thereby is not to be disregarded. The little kindness is also the moral cornerstone for the construction of a harmonious society; it is very much like the fireflies--- each individual has a very limited illumination, but thousands gathering could light the night sky