Christianity came first to the continent of Africa in the 1st or early 2nd century AD. Oral tradition says the first Muslims appeared while the prophet Mohammed was still alive (he died in 632).
Thus both religions have been on the continent of Africa for over 1,300 years. Some would argue that both Islam and Christianity are indigenous African religions. Not everyone shares this view.
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Certainly the first Muslim teachers and Christian missionaries had little respect for the traditional religions they came across. Both Islam and Christianity are religions of the book; their doctrinal authority lies in their scriptures.
African traditional religions produced no written works, but derived their authority from oral history, custom and practice, and the power of priests, kings and others gifted in dealing with spiritual issues. This lack of scriptures led to the assumption that people in Africa were not capable of 'proper' religious observance. But some European missionaries and explorers were struck by the intense spirituality of Africans.