The description of how Christianity reaches Okonkwo's tribe I imagine to be a fairly standard description of how Christianity spread amongst the so-called "heathen" tribes of Africa. We are told that after Okonkwo went into exile after accidentally killing a kinsman, six missionaries, under the headship of a man called Mr. Brown, travel to Mbanta. They have an interpreter called Mr. Kiaga who helps them to communicate to the villagers and explains to them that, in their opinion, the gods that the tribe have been worshiping for aeons are not true and that polytheism is wrong. Mr. Brown tries to accomplish his task through mediation and communication, not setting out to provoke the tribe or anger them.
Soon, however, Mr. Brown falls ill and is replaced by a very different kind of missionary, Reverend James Smith, who is much more strict and believes in a black and white approach to religion and belief. This leads very quickly to a conflict between the Christians and the tribe that propels the plot to its tragic conclusion.
The two missionaries therefore symbolise two approaches: one of accommodation and one of conflict. It is the second that of course shows itself to be most effective, if only through the way in which it provokes the Africans to respond and therefore allows the whites to crush them.