Complex sentence: Christianity, a relative newcomer introduced in the late 19th to mid 20th centuries with British and French colonisation, has become the predominant religion in the central and southern part of Nigeria, and the coastal regions stretching from southern Ghana to coastal parts of Sierra Leone.
Split into simpler sentences: Christianity, a relative newcomer introduced from the late 19th to mid to late 20th centuries, is associated with The British and French colonisation eras. It has become the predominant religion in the central and southern part of Nigeria, and the coastal regions stretching from southern Ghana to coastal parts of Sierra Leone.