In terms of what we see in Harper Lee's To Kill a Mockingbird, I can believe that Lee would agree with the statement, "Negroes are more sincere Christians than whites."
There are several reasons for this. The white community, while proud of their heritage and the "civilized" way they live, do not reach out with Christian charity to their neighbors. Miss Stephanie, for example, is a terrible gossip and will talk about anyone. The "foot washers" who pass by and condemn Miss Maudie for her flowers, warning that she will go to hell, have no concept of Christian love or charity. Though the women gather at the Missionary tea with Aunt Alexandra, thinking that they are helping the people they are supporting in Africa with Christian charity—they cannot see the irony that they are willing to do for the needy blacks in another country, but have no compassion for the blacks in their own community. Racial prejudice within these white creates a line that not even Christians can cross. In the face of Tom Robinson's trial, when the evidence clearly shows that Tom is innocent, the jury will not look at its conscience but only the color of Tom's skin—except for Cunningham.
However, the black community watches out for its own in taking care of Helen Robinson while Tom is in jail: they take up an extra collection in church for her. They come to the courthouse, not because they are curious, but because they are there to support one of their community. They respect Atticus because of what he has tried to do for Tom, and do not discriminate against him because of his color—as most of the white community does against the blacks.
There are a few exceptions in terms of the white community: Atticus, Miss Maudie and Link Deas act like Christians with others, even those who are black. However, for the most part it seems that the black community expresses Christian love, while the white community sees only a "racial divide."