Obierika is filling in Okonkwo, the novel's hero, on the latest troublesome doings of the missionaries in Igbo land in Nigeria.

"When nearly two years later Obierika paid another visit to his friend in exile the circumstances were less happy. The missionaries had come to Umuofia. They had built their church there, won a handful of converts and were already sending evangelists to the surrounding towns and villages. That was a source of great sorrow to the leaders of the clan; but many of them believed that the strange faith and the white man's god would not last. None of his converts was a man whose word was heeded in the assembly of the people. None of them was a man of title. They were mostly the kind of people that were called efulefu, worthless, empty men." (Chapter 16)

In this passage, we see the conflict between colonial and indigenous culture beginning to play out. We see that the arrival of the missionaries—agents of colonialism in the novel—brings "sorrow" to Okonkwo's people. We see how their influence and control is quickly spreading over Igbo society. Of course, this conflict, and colonialism in general, is the big theme of postcolonial literature.

What's important about this passage, and about Things Fall Apart as a whole, is that it's telling the story of colonialism from the perspective of the colonized. In this passage, the emphasis on the clan elders' "sorrow" over the arrival of the missionaries, for example, tells us that colonialism wasn't this wonderful thing it was cracked up to be by the colonizers. Colonizers often thought they were "enlightening" the colonized (through religion, for example), but in reality, the colonized had their own (often highly developed) cultures and religions. Colonialism changed people's cultures; it divided people. Even in this brief excerpt we see how colonialism is already breaking up the community.