I have not read the book, but I think I can help you to determine the answer. First, the theme is the lesson or the moral. It is universal, so do not mention any names. For example, because the church takes in the rejects, a possible theme could be, "one person's trash is another man's treasure". Does that make sense with what you are reading? What is it about Nwoye that makes her/him unlike the others? What don't they like about the character that makes her/him an outcast? Lastly, treatment of outcasts can often be bullying, terrorizing treatment (like threats), ignoring someone. Does any of these define the treatment of the missionaries by the clan?