First, I need to let you know that I changed the spelling of "crustians" to "chrustians" (as it is in the text).
Manley Pointer, in "Good Country People", refers to Christians as "Chrustians". The reasoning behind this can be answered in a few ways.
First, dissect the word. Crust (chrust) means a tough or hard outer layer. "Ian" is either a suffix that refers to what someone is or does OR is used as a suffix that forms an adjective or noun. Therefore, "chrustian" could have two meanings based upon this: a person with a hard outer layer or a word which describes a hard person.
Second, the story is written using southern dialect. The use of the word "chrustian" could simply be Flannery O'Conner using the dialect of the area from which she bases the story: Georgia.
Lastly, Pointer could simply be making fun of Christians. By the end of the story, readers find out that Pointer is not a bible salesman, but he seems to prey on them based upon their unshakable trust of other "Christians".