[General] A sentence having, I guess, a sequence of tense problem.

[infiniteone]

A: All that is necessary for evil to prevail, someone once said, was for good people to do nothing.

A':All that is necesarry for evil to prevail, someone once said, is for good people to do nothing.

A'':All that was necesarry for evil to prevail, someone once said, was for good people to do nothing.

B: All that is necesarry for evil to pravail is for good people to do nothing.

C: Someone once said that all that is necesarry for evil to prevail
is for good people to do nothing.

C' : Someone once said that all that is necesarry for evil to prevail
was for good people to do nothing.

C'' : Someone once said that all that was necesarry for evil to prevail was for good people to do nothing.

A is a sentence I found in a book. But it sounds ungrammatical to me.
I think If C' is right, A should be ok too. Is it right?
And I want to know the diffrence between C and C'' in terms of meaning.
maybe with that, I will be able to presume about A and A'' .


Things regarding tense are always difficult for me.
Could you help me? Thanks for reading.

 

[Heterological]

You're right; A is not grammatically correct. The tense of "say/said" is irrelevant; the two forms of "be" need to agree. The difference between having them in the present (All that is necessary for evil to prevail is for good people to do nothing) and the past (All that was necessary for evil to prevail was for good people to do nothing) is that when we use the past, we indicate that evil has already prevailed because good men did nothing, while the present tense tells us that evil might someday prevail if good men do nothing, but it has not yet.