The theme is grace, according to the author herself--the idea that nothing we do can save ourselves from our own faults, that only God's grace can do it.
Thinking specifically of the story, each member of the family has specific faults. It could even be argued that they have specific sins--the mother has sloth, the father anger, the grandmother vanity, the children envy, etc all except the baby
and nothing any of them do in the end saves them from the place their combined actions have brought them to
especially the grandmother, who has the most obvious faults of character and who tries the hardest to save herself by pleading, reasoning, etc
Notice the way the grandmother reaches for The Misfit at the end? This is not some desperate attempt to save her skin but is actually the moment of grace, the act of love that will save her (in O'Connor's sense of save, ie in God's eyes not in the sense of saving her life, obviously) This is what The Misfit means when he says she would have been a good woman if she'd had someone there to point a gun at her all her life. It is her own moment of acceptance--and act of love--the message is that, no matter how terrible your deeds, no matter how rotten your spirit is, redemption is possible through the Grace and love of God.
Source(s):
The two A papers and extensive research I have done on O'Connor's stories. Also, the author's own thoughts on the story come from letters printed in a book by Frank Asals and from other sources, including The Compact Bedford Introduction to Literature.