Complex sentence: Xavier believed he was only a channel for the work of the spirits; that he was not able to produce any miracle, such as healing people, and he could not contact someone that was dead, unless that person was willing to be contacted.
Split into simpler sentences: Xavier believed he was only a channel for the work of the spirits and that he was not able to produce any miracle such as healing people. He believed he could not contact someone that was dead, unless their spirit was willing to be contacted.