There's a fascinating article in Christianity Today about a new and extensive study on the effects Christian proselytizing missionaries had on the societies they served. The detailed research contradicts the commonly held view in academia these days that missionaries were detrimental colonialists. Rather, the missionaries who went as agents of the church, not the government, with the Gospel message had significant positive and lasting influence on societies.
One example is John and Alice Harris who went to the Congo as Baptist missionaries at the turn of the last century. The couple photographed the human rights abuses of the Belgian colonial government who enslaved and exploited the native people for the rubber and ivory trade. The Harrises traveled Europe and the US exposing these actions, which led to reform in the Congo. John Harris campaigned against colonialism and in favor of self-determination for native people.
A new exhibit in England shows some of their photographs.