The answer to this question can be found in Chapter Eighteen, which describes the first outcast of the tribe to join Mr. Kiaga's church. Note what Mr. Kiaga says to this first outcast, and the conditions that he places on his conversion:
"Unless you shave off the mark of your heathen belief I will not admit you into the church," said Mr. Kiaga. "You fear that you will die. Why should that be? How are you different from other men who shave their hair? The same God created you and them. But they have cast you out like lepers. It is against the will of God, who has promised everlasting life to all who believe in His holy name."
Thus Mr. Kiaga's actions and words reveal his rejection of traditional tribal culture and the way that some members of the tribe are branded as being outcasts. He presents a message of new hope, based on Christianity, where those who are rejected by men can be accepted by God if they take the step of defying the norms that have oppressed them for so long and shave off their hair, which has served to mark them as being separate.