Achebe's beliefs are constant as they are revealed throughout the whole novel. In chapter 16, two important issues are brought up by Okonkwo and Obeirika's conversation. First, Achebe points to the fact that change is inevitable (tradition vs. change) by discussing the coming of the white missionaries, and second, he deals witht the conflict of Christianity vs. the Ibo religious practices.
In chapter 16, Okonkwo can't wait to get back to his village to chase the missionaries away - he is battling the changes taking place. Achebe believe that change is inevitable and adapting to change is essential. Achebe demonstrates this through Okonkwo's character who is not able to adapt and eventually dies as a result.
Also in chapter 16, Obierika tells Okonkwo of the white missionaries religion which seems humorous and offensive to most of the Ibo tribe. The message the missionaries deliver sounds contradictory, and the missionaries insist that the Nigerian beliefs and customs are bad. Achebe grew up in a Nigerian village, but he also had Christian parents so he witnessed this struggle between 'old' religion and 'new' religion first hand. He believed that Christianity could and should be introduced without denouncing Ibo culture, but he also saw that this rarely ever happened.