In "Barn Burning," what is the importance of this quote?Later, twenty years later he was to tell himself, "If I had said they wanted only truth,...

In this particular quote, Sarty doesn't feel the need to
answer right away (although, twenty years is a long time to keep silent).  Faulkner
allows Sarty to use the text as he chooses, revealing Abner's guilt on his own terms,
yet remaining loyal to Abner throughout.   If you notice, adult Sarty does not live in
the present--he carries no real voice.  Instead, the narration becomes an endless cycle,
much like the cycle of abuse, in which Sarty focuses on telling the truth about
Abner. 

More importantly is what isn't
said in those twenty years. As an abuse victim, the silence corresponds to the
symptomology of repression or a refusal to 'feel."  Abner has a history of physical
abuse and Sarty knew that any response was the wrong one.  As a child, Sarty would only
know Abner's behavior as an expression of 'love;' it wouldn't be until he grew into
adulthood that Sarty would understand Abner's need to dominate and control his
family. And, as in many cases of abused children, it often takes years for the victim
(Sarty) to recognize what that violence has done.   So, Sarty is rendered voiceless for
twenty years, powerless in his attempt to control the abuse--and in his abilitiy to
voice it. 

Because Sarty know exactly how to act/appear in
front of his father, he also recognizes that:

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It is not in defense of this violence but in an
effort to understand it—the 'savage blows ... but without heat' that I might add that Ab
seems here to be passing on, in a more explicitly despotic, violent form, the
naturalized, axiomatic social and economic violence he feels directed against
himself"(emphasis
added). 

The
final understanding is that Sarty must have some control over his situation, his
powerlessness, and his anger at Abner.  Perhaps on some level it's what compels Sarty to
testify against his father.  As a child he his moral compass was black or white, right
or wrong; he had no understanding of justice and vendettas.  For Sarty the child, this
amounts to a Catch-22...how do do the right thing, without angering the man who provides
for his family.