The best way of answering this question is by refering to the influence that Christianity is having on Ibo society and on Okonkwo himself. As it slowly but pervasively enters this tribe's life, the divisions and problems that it creates become bigger and bigger. This is something that is recognised by the tribal elder who delivers the speech at the end of Chapter Nineteen who foresees the massive problems that will be caused by the presence of Christianity amongst them:
But I fear for you young people because you do not understand how strong is the bond of kinship. You do not know what it is to speak with one voice. And what is the result? An abominable religion has settled among you. A man can now leave his father and his brothers. He can curse the gods of his fathers and his ancestors, like a hunter's dog that suddenly goes mad and turns on his master.
This elder clearly explores the threat that Christianity represents. In Ibo society, part of what holds it together so tightly is the fact that they are all in agreement on a set of tribal practices and beliefs. Now, suddenly, that agreement has been lost, and as a result, what is holding the tribe together is beginning to fade away.
For Okonkwo, personally, all of his hopes and dreams and what is most important to him--being strong and unfeminine--is at risk through his son Nwoye and the way that he has been attracted by Christianity. His whole world and everything he stands for is falling apart through his son's conversion to this foreign religion.