In chapter 16 of Things Fall Apart, the missionaries come to the village of Mbanta, where Okonkwo is spending his exile. At first, they are met with curiosity and amusement. One of the missionaries is a white man, so most of the village flocks together just out of interest, to see him. When the black missionaries begin to tell the people of the Christian faith, most of them consider the words foolish. The ideas of Christianity are extremely weird to Ibo people. The strangest of all of those is the notion that men are equal and are viewed in the same way by the new god.
There are many reactions from the listeners. Most of the village believes the missionaries to be harmless fools who are severely misguided. In their mind, they have seen the work of their own gods many times, and the idea of anything else sounds ridiculous to them. Okonkwo and possibly some others are more angry than amused. Okonkwo himself believes that they should deal with the missionaries quickly, firmly, and immediately. In a way, the novel proves him right, because the Christians continue gaining power and end up all but destroying the previous societal order.
Not all of the Ibo people are sad about that, however. Okonkwo's son Nwoye has felt completely out of place for a long time. He has had to sit through the death of his friend Ikemefuna and listen to the sad cries of twins being taken to die in the bushes. So when someone comes to say that all of it is wrong and his gut feeling has been right the entire time, it's only natural that Nwoye is drawn to that. The Christians preach about brotherly love between all men and against inequality. Most importantly, Nwoye is one of the first to feel that there is an alternative to the brutality he has seen. Nwoye feels relieved, but also confused:
The words of the hymn were like the drops of frozen rain melting on the dry plate of the panting earth.
As the novel progresses, you might see that what truly brings converts like Nwoye to the new church is their confidence. The Ibo people were first enthralled by their beautiful singing and talk of brotherhood, sure, but what made them convert was something else. A few chapters later the novel says, speaking of one of the missionaries:
Mr. Kiaga stood firm, and it was his firmness that saved the young church. The wavering converts drew inspiration and confidence from his unshakeable faith.
I believe that casts a revealing light on those first positive responses as well. It's not even what the Christians are saying at first, although the outcasts and the outsiders are drawn to a faith that accepts them. It's also that they say everything confidently and firmly. It helps their message in a society that doesn't allow doubt and interpretation. For example, Nwoye has always been forced to believe that there's something wrong with him. Now, finally, he sees others—and not weaklings, not children, but grown men and women—propose a different way of living. Therefore I would say that the first act of the missionaries that evokes a positive reaction from some of the Ibo is that they don't back down in the face of opposition by the majority of them.