There is some evidence that Mary Grace is being used as a vessel from God. First of all, it does seem as though Mrs. Turpin is not one who would be normally listening to someone like Mary Grace. All Mary Grace did was throw the book at her, and she seemed to fall into a trance-like state.
When the girl talks to her, she is desperate to hear what is said as if it were a “revelation.”
There was no doubt in her mind that the girl did know her, know her in some intense and personal way, beyond time and place and condition.
Another aspect of the revelation is her reaction to it.
I am not," she said tearfully, "a wart hog. From hell." But the denial had no force. The girl's eyes and her words, even the tone of her voice, low but clear, directed only to her, brooked no repudiation.
She tries to deny the fact, but she thinks that the girl knows hew on a deeper level, and therefore cannot be wrong.
Mrs. Turpin seems to wonder at first why she was the one who was chosen.
"Why me?" she rumbled. "It's no trash around here, black or white, that I haven't given to. And break my back to the bone every day working. And do for the church.”
When she sees the vision of ascendency to Heaven, and hears the cricket chorus of hallelujahs, she seems to have accepted the “abysmal” revelation and is irrevocably changed.
Messengers in myths often come in the form of unexpected persons, like Yoda in Star Wars. They test their pupils by taking unusual forms and looking humble. It does make sense that God would choose a fat college girl, who has no other real power.