On the first day of the trial of Tom Robinson, people from the south end of Maycomb County stream through town as though in a parade. When one wagon pulled by a team of mules slows down by Miss Maudie's house, "a shrill-voiced woman" calls out, "He that cometh in vanity departeth in darkness!" Miss Maudie retaliates with another line from Scripture: "A merry heart maketh a cheeful countenance!"
The woman who has shouted the first line of Scripture is what Miss Maudie terms "a foot-washing Baptist." This is a strict, Puritanical person who believes that anything that is pleasurable is sinful. Miss Maudie's scriptural retort implies that what brings people joy is actually acceptable to the Lord since it is mentioned in the Bible (Proverbs 15:13). For, one who has a "glad heart" has a strong spirit. Here is a modern translation of this passage:
A joyful heart makes a cheerful face, But when the heart is sad, the spirit is broken. --New American Standard Bible
In other words, Miss Maudie tells the "footwasher" that all her fundamentalist negativity breaks the spirit and leaves people without hope and happiness. But, by having such lovely flowers, Miss Maudie derives joy and a "glad heart" from her garden, thus renewing her spirit.