Atticus knows even before he accepts the case that it will be a losing cause, explaining to his brother, Jack, that
"The jury couldn't possibly be expected to take Tom Robinson's word against the Ewells."
In 1930s Alabama, a white man's word was always taken over the word of a black man. Although Atticus knew he couldn't win the case (hoping that an appeal might later set Tom free), he meant to try his hardest to gain an acquittal. Judge Taylor selected Atticus instead of the normal public defender because he wanted Tom (who the judge may have recognized was probably innocent of the charges) to get the best defense possible. The men of the Idlers' Club knew that Atticus was the best attorney in town, and they knew that he would not just go through the motions as a public defender might have done: He would try and prove Tom innocent by any means possible, even if it meant disgracing Tom's accusers, Bob and Mayella Ewell. That is why Billy (one of the Idlers) responded that
"... Atticus means to defend him. That's what I don't like about it."
He recognized that if anyone had even the slightest chance of breaking new judicial ground--convincing an all-white jury to acquit a black man of raping a white woman--Atticus was that man.