It is a great irony that over the last couple of hundred years, some Christians and some Muslims have made it easier for people to convert to the other faith.
Speaking of the expansion of the Muslim Fulani Empire, the destruction of the Old Oyo Empire that led to the Yoruba wars, and the subsequent influx of Egba people into the coastal regions of Nigeria in the early 19th century, Ajayi contends that ‘[i]ndirectly therefore the last advance that Islam made to the south prepared the way for the advance inland of the influences of Christian Europe.’
[J. F. Ade Ajayi, Christian missions in Nigeria, 1841-1891: the making of a new élite (London: Longmans, 1965), p. 23.]
Meanwhile…
‘Curiously enough, under the British empire, and as a direct result of British policy, Islam received indefinitely more converts than the jihads had ever brought it. It is also true that the British Raj assisted in this respect by Christian missions, produced the conditions for a reformation and revitalization of Hindu faith. In the end, is it possible that the other faiths of the world, more than Christianity, will prove to be the real beneficiaries of the Imperial period?’
[Andrew F. Walls, ‘British Missions’, in Missionary ideologies in the imperialist era, 1880-1920, eds Torben Christensen and William R. Hutchison (Århus, Denmark: Aros, 1982), pp. 159-65, p. 165].