Changing of the Guard
During the time that The Canterbury Tales was written, England was going through a large political and social change. The long-held traditions of religious piety and the feudal system had been radically altered when an epidemic of the bubonic plague had reduced the population severely. Therefore, a society that kept all but the richest of its subjects in servitude to the land and kept them in line by fear of the Christian Church began to fall apart, with many religious wars, and more importantly in Chaucer, the emergence of a middle class. The Middle class is very important because they tend to question long-held beliefs of a single moral standard and the validity of religion in their lives. The Knight is one of the only characters who comes from a noble position, and keeps to the old ideals of chivalry and fairness. It is fitting that he tells the first story of the Tales, almost as an epilogue to an era instead of a prologue to Chaucer's stories.
People gained a freedom to think more for themselves than they ever had before, and so different theories about what is right and wrong and best and worst come up, and are discussed, evaluated, and often argued vehemently in Chaucer's Tales.
Saintly or Sexual?
Chaucer plays with the concept in religion in The Canterbury tales. He shows that many clergymen are corrupt, both by the stories that the pilgrims tell and the pilgrims themselves, for example the Summoner and Pardoner. In fact, most of the characters of The Canterbury Tales have a loose interpretation, or no interpretation, of morality as told by the Bible. The Sailor, the Reeve, and the Miller tell racy bar-room stories, and the Wife of Bath speaks of her many husbands, and many years of marital experience, in a way that puts quite a different interpretation on religious and moral beliefs of her day.
Characters that keep to a strict moral compass are few; the Prioress and her company are the most notable examples. However, even they have an altered view of morality. In the Prioress' Tale, she exalts a young boy as a martyr, but condemns the entire Jewish Race in the same breath. Historically, this was an accepted thing for Christians to do, but Chaucer puts the story there as a subtle attack on the intolerance of Christianity.
Crime and Punishment
In each story of The Canterbury Tales, a punishment, revenge, or retaliation always occurs. However, the nature of what should be punished varies from character to character more than any other theme in the book. As was mentioned above, the Prioress considers cold-blooded murder of an innocent, or more generally, any attack on Christianity, a sin that must be punished. The Friar gives a litany of stories where great rulers were destroyed because of their greed, arrogance, or ignorance. On a much lighter side, Promiscuity is rarely seen as a sin to be punished, but as a punishment for some other sin. For example, in the Reeve's Tale, the Character of the Miller tries to rob the two students of their grain, but they get the best of him by sleeping with his wife and teenaged daughter. In many other stories, husband's inattention is reason enough for them to be punished by their wives infidelity.
Wives and Husbands
Another theme of The Canterbury Tales is what makes a good wife or husband. The stories range from ones that empower the wives, like the Wife of Bath's Tale, and at the other spectrum, the Scholar's tale, where the wife endures patiently and happily sadistic griefs that her husband tests her with, such as pretending to kill their children and pretending to divorce her.