Francis, in the end, surrenders to his fate and takes the first steps on the path his great-uncle had prepared him for - to be a prophet of God.
At the end of the novel he unwittingly performs the action that confirms his status as a prophet, and he finally accepts his role as a messenger of God.
After spending an extremely difficult week fighting this fate, Francis (Tarwater) finally gives up the fight. The explanation and answer to the question, "Why does he do this?", is ambiguous but may ultimately be simple.
Tarwater, like his uncle and his great-uncle, has something in him that might be called his "nature", which cannot be removed.
This internal force is seen as a madness by the uncle and as foolishness by Tarwater. Yet, this force is an unrelenting impulse and it drives Tarwater, finally, to accept himself as a prophet. All of his efforts to evade this fate (and to evade his nature) have failed, though they have been extreme.
He is violent, and a great deal of violence is done to him—he sets fire to a house, he murders a boy, he is raped.
None of the events have the power to remove the internal force that Tarwater feels as a hunger inside him.
Increasingly hungry and unable to eat anything due to an incipient and overwhelming nausea, Tarwater's hunger takes on a symbolic significance that is clarified in the novel's conclusion.
At the end of the novel, Tarwater has a vision of a multitude—his great-uncle among its number— being fed loaves and fishes by Christ from a single basket. It is only then that he becomes aware of the object of his hunger and realizes that “nothing on earth would fill him.”
Tarwater is powerless to change his course. His efforts to do so have failed entirely and his final vision serves to confirm the fate that his great-uncle had predicted and created for him. In this argument, the mechanism of Tarwater's acceptance of God is clear, though the reason for this course of action remains open to interpretation.
Scholars of O'Connor suggest that the novel should be read from a religious-symbolic perspective, that O'Connor was working to convey a specific message with the novel, and that Tarwater's final conversion is the inevitable step in the achievement of the author's aim.
O’Connor is also interested in making fresh for modern readers the idea of the prophet. Her prophets are individualistic, anti-intellectual, destructive, and suicidal—so driven by unconscious forces as to seem insane.
We might argue then, if somewhat elliptically, that Tarwater accepts God because he is a prophet. This is his nature and his revelation confirms that he is defined, not by his own will, but by the will of a power that resides in him.