What is the theme of chapter 11?

Chapter Eleven's main theme is that of Tribal belief.

Particularly since one of the threats to Igbo life is the coming of the new religion, tribal belief is a theme of some importance. Igbo religious beliefs explain and provide meaning to the world; the religion is also inextricable from social and political institutions. Achebe also shows that Igbo religious authorities, such as the Oracle, seem to possess uncanny insights. He approaches the matter of Igbo religion with a sense of wonder.