Complex sentence: Some people, mostly children, use the gesture to excuse their telling of a white lie; this use has its roots in the belief that the power of the Christian cross might save one from being sent to hell for telling a lie.
Split into simpler sentences: Some people, mostly children, also use the gesture to excuse their telling of a white lie. This may have its roots in the belief that the power of the Christian cross might save one from being sent to hell for telling a lie.