What is the theme in The Story of Mankind by Hendrik Willem Van Loon?

One of the primary themes throughout the novel is that of religious persecution. Throughout history, religious differences have caused mayhem throughout the world, as different belief systems clash and compete for power. From the earliest priests, those of the religious hierarchy have been seen as keepers of knowledge and as leaders of the people. By the fifth century B.C., Christianity is beginning to take hold as rulers learn that by teaching barbarians Christianity, they can begin to control them. By 1095, the Crusades begin. Whereas other areas of history have experienced smaller bouts of religious fighting, the Crusades are the first real attack for religious cause.