The missionaries were motivated by belief that African natives needed the Christian message to be saved eternally, as well as feeling motivated to help the earthly well-being of the peoples in Africa as a Christian duty to help those in need.
Note that there is an implication in all of this that the European culture was superior to the African culture, and that the people of the central African rainforest were indeed in need. Missionary endeavors set up medical clinics and schools and other facilities to aid the African natives. There certainly was much good done in this way, but there also was a paternalistic attitude of the "white man's burden" to help seemingly lower human beings. (Rudyard Kipling wrote the poem "White Man's Burden" in 1899 in reference to a different imperial venture, but the thought applies here as well.)