"A Good Man is Hard to Find" is a timeless story born from the newly emerged 1950s Southern American culture which was becoming secularized in its religious values, and influenced by the "3 Ms": mobility, mass media, and materialism.
O'Connor uses the Misfit's fiery sermon against the grandmother's "watered down" religious values as a mouthpiece for her denunciations of what Ralph C. Wood calls the "newly emerging American civil religion [which melted] particularized historic faiths into thin religious gruel." He says O'Connor "likened such saccharine religion to pornographic literature: the achieving of cheap and easy ends at the expense of valuable and difficult means." Let's face it, the grandmother is a hypocrite, a self-righteous back-slider, one who takes her salvation for granted because she hasn't had a Misfit's gun (a symbol of suffering and death) to her head every moment of her life.
O'Connor also uses the car as a secular symbol for the religious state of the grandmother. Indeed, she is lost and can't remember if her old home is in Tennessee or Georgia. So, the 1950s mobility and culture of the car is a metaphor used to show how displaced Americans had become (a recurring theme in O'Connors stories--also read "The Displaced Person"). Needless to say, car wrecks and broken down clunkers are common in her work.
The car, newspaper clippings, the contents of her purse, and the grandmother's cat worry her more than the lives of her family and soul, showing a growing sense of 1950s materialism and mass media messaging--all of which take away from the grace and revelation of belief. The grandmother is more worried about her possessions than she is about her own soul. The Misfit is a reminder that she can't take her possessions with her (after death).