What does Atticus say God is? How is this different from what foot-washing Baptists believe?

From what I
can see, you are referring to what Scout says about her father's definition of God in Chapter 5.
In that chapter, Scout and Miss Maudie are discussing religion.

According to
Scout, her father's definition of God is "loving folks like you love yourself." This
corresponds with the Golden Rule, where we treat others the way we would want to be treated.
Atticus' definition of God and true religion, however, differs from that of "foot-washing
Baptists."

If we refer to the text, Miss Maudie defines
"foot-washing Baptists" as legalists who strictly adhere to the letter of the law.
According to Miss Maudie, foot-washing Baptists are "literalists": they are so focused
on the academic definition of morality that they forget to display the true spirit of good
religion. To Miss Maudie, foot-washing Baptists are so busy "worrying about the next world
theyve never learned to live in this one."

For his part, Atticus is
first and foremost concerned with the Golden Rule. In Chapter 11, he tells Scout why he is
defending Tom Robinson.

"This case, Tom Robinsons
case, is something that goes to the essence of a mans conscienceScout, I couldnt go to church
and worship God if I didnt try to help that man.

Atticus
is primarily motivated by his conscience. He defends Tom because he believes that it is the
right thing to do. To Miss Maudie, such a person is closer to God than literalists such as
Stephanie Crawford.