What was it about Nwoye's actions in chapter 14-19 that disturbs Okonkwo?

Nwoye was seen associating with the Christians. After being threatened by Okonkwo, he joins the Christians and volunteered to go to Umuofia where a school had been set up to to teach young Christians literacy. Effectively, Nwoye had abandoned the gods of Okonwko and his fathers.