It is natural to have different beliefs, whether we're talking about one religion opposed to another religion or about different beliefs or opinions between members of the same religion. Divisions occur when members of one religion are not prepared to accept the presence of poeple who follow different beliefs. It is natural not to agree one with another, but one must be prepared to tolerate the other. In that sense it is not much different from power struggles between poeple of different countries or political views. It all comes down to the ability to be able to resolve conflicts with the required opennes and tolerace toward the other party.