You have asked a fascinating question, and tracing the development of this key character casts light on some of the main themes of the novel and the author´s hope for South Africa, in spite of all its problems and issues.
Stephen Kumalo, who is named after the martyred Saint Stephen in Acts in the Bible, starts off as a Reverend who is working in his own rural village of Ndotsheni. His trip to Johannesburg reveals how ignorant and naive he is about the city and the way things work here, and above all, the racial tensions that divide his country so dramatically. Kumalo is easily amazed by such sights as an electric train:
Here is a white man´s wonder, a train that has no engine, only an iron cage on its head, taking pwer from mtal ropes stretched out above.
Note the basic and simplistic way this electric train is described - Kumalo is seeking to put into his words the marvels that he sees. He is obviously a country man and is amazed and overwhelmed by the complexity of the city. Consider the following quote, when he finally reaches Johannesburg:
He sees great high buildings, there are red and green lights on them, almost as tall as the buildings. They go on and off. Water comes out of a bottle, till the glass is full. Then the lights go out. And when they come on again, lo the bottle is full and upright, and the glass empty. And there goes teh bottle over again. Black and white, it says, black and white, though it is red and green. It is too much to understand.
Kumalo is not able to understand all the wonders that he sees or process the realities of city life. This results in him being ripped off easily at the bus station.
However, as he begins to search for his son and discovers what has happened, Kumalo begins to learn about the harsh realities of his people in the city and has to confront not only the sins of his family in Gertrude´s prostituion and his son´s murder, but also the sin within himself. Think of how he threatens and bullies his son´s partner. He experiences times such as in Chapter 13 when he visits Ezlenzeni with Msimangu when his doubts threaten to overwhelm him, yet manages to find faith in and through his doubts.
He also encounters and is inspired by the various efforts that are trying to improve the lot of his people. He admires the organisation of the bus boycott and is impressed by the character of Dubula. Likewise the organisation in the Shanty Town helps him to see that his people are trying to make the best of a bad situation.
Through confronting such failings in his family and in himself, yet also the positive aspects of this terrible situation, Kumalo is able to return to Ndotsheni a humbler, but above all a wiser man who realises that in the face of the massive issues confronting South Africa you cannot be passive nor simply trust in God to solve all the problems. Thus he is inspired to action, not simply accepting the degredation of his people and land meekly.
Kumalo then is inspired and changed through his journey to "re-build the tribe" both by trying to restore his family, his tribe and his land. Although he does not achieve full success in any of these areas, he is trying to make an albeit limited difference in the way he can. He is no longer ignorant, naive or passive, and is able to minister more effectively to his village because of the experiences he has undergone.