Mennonites are a religious group similar to the Quakers, Shakers, and Amish religions. Mennonites are allowed the luxury of electricity, in comparison with other more conservative denominations that don't allow use of power.
They believe that bright colors are boastful, or are a display of selfish pride, and Miss Maudie's colorful flowers are considered sinful by them. They yell a scriptural verse to her dealing with the issue of pride and sin, and in return, she rebuts their statements with a bible verse of her own, which contradicts their belief.
Mennonites are like the amish but the mennonites use electric things . they didn't like the way Ms. maudie was doing something.