The structure of the novel might be described as the reversals of belief in Jude and Sue and their changing marital relationships as they both go down to defeat. In the beginning Sue's view of things is secular and rationalist, expressed, for example, in her sympathy with ancient rather than medieval culture, her scorn of conventional religious belief, her buying of pagan statuary, her reading of Gibbon. Jude's beliefs are, at first, conventionally Christian, as his desire to be ordained, his reading of standard authors, and his love of medieval culture and architecture show. By the end of the novel Sue has reverted to conventional beliefs, as evidenced by her concern for the sanctity of marriage and her desire to perform penances for her sins. On the other hand, Jude no longer professes his old beliefs and finds himself, as he says in his speech to the street crowd in Christminster, in "a chaos of principles."
This change in beliefs is closely paralleled by their marital relationships. At first, they are separated by marriage to other people as they are apart in belief As Jude's ideas change, they are legally freed by divorce, and they come to live together and to be "married," in fact, if not in name. When Sue returns to conventional Christian beliefs, they separate and remarry their first spouses.
Jude's death as a failure in Christminster and Sue's forcing herself to go to Phillotson's bed are striking signs of their defeat in life. This defeat is mirrored as well in Phillotson, who at Marygreen has fallen to the bottom professionally and who stiffly requires Sue to swear loyalty to him on a New Testament, and to a lesser extent in Arabella, who though she loses Jude does not lose her vitality.
In these changes and defeat Hardy has embodied the theme of his novel: Jude and Sue have been caught up in the modern spirit, have struggled to break free of the old ways, and have suffered and failed. It is this that justifies Hardy's description of the novel, in his preface to it, as a "tragedy of unfulfilled aims."