In chapter 31, what is the sentence that forms the crisis or turning point of the novel?

The sentence that is the turning point is the following. It expresses what Huck thinks as he decides to tear up the letter he has written to Miss Watson revealing the whereabouts of Jim:

“All right, then, I’ll go to hell”—and tore it up.

In the upside-down moral world of the pre-Civil war South, Huck believes he is a sinner because he is concealing Jim—who his culture considers property—from his owner and helping him to escape. His decision not to expose Jim is a turning point because it represents Huck thinking for himself and developing his own moral compass. Before this, he has simply accepted the morality that was passed down to him. Now, however, he stops to consider all the acts of kindness Jim has done for him. He stops sees Jim as merely a slave and perceives that he is a real human being. At this point, Huck accepts Jim's full humanity—even if the price of protecting him is going to hell.