The Wife of Martin Guerre is rich with themes and topics worthy of analysis. This is because the novel achieves the task of using multiple perspectives under which specific social issues are brought up.
Yet, there is one theme that conquers all others: loyalty. The theme of loyalty is so multidimensional that it spreads out into several sub-themes that include family roles, feudal feuds, love, marriage as business, Catholicism, betrayal, adultery, male and female roles, sexuality, chauvinism, identity theft, gender injustice, punishment, and forgiveness. All these themes surface in the story but only under the umbrella of "loyalty" as the main theme. This is because loyalty is the central causative factor of the action in the novel. It certainly is the main catalyst of it all.
Martin's loyalty to his family contends that he would marry at a young age, as other boys would have, to preserve the family's finances. However, Martin lies, escapes, and breaks from those binds. It is the need for loyalty, and the disregard for it, that brings the problem of the story, and that puts every character to the test.
Loyalty also manifests in the character of Bertrande: the poor woman who not only gets abandoned by her husband, Martin, but who also has to endure the entrance of an intruder, claiming to be Martin, in her marital bed. Fortunately for Bertrande, she found joy (both love and apparently sexual satisfaction) in this man whom Martin's own family decided to treat as the real Martin. However, when the real Martin Guerre comes back, Bertrande's loyalty to the man she actually married, and the man she now loves, will put her in a dire situation.
Moreover, Martin's family's loyalty to Martin is so blind and ridiculous that they accept the intruder to be Martin, at face value. This looks like a twisted act of loyalty that only people of their kind could understand. Yet, when the real Martin appears, the family's loyalties switch, as if by magic, and they return to their real Martin gladly. This is what puts Bertrande in the position of a potential adulteress: because, after all, it is Martin's family who demands that she takes in the intruder at face-value, as well.
Finally, Martin's own loyalty to Bertrande is also lacklustre. He returns after having abandoned her and those who love him. Instead of looking at the big picture, he accuses Bertrande of actively choosing to accept the intruder, which would make her an adulteress. This is unfair because it is his family who demands that she does as they tell her, which is to accept the intruder. Hence, although Bertrande is simply trying to do the right thing, nothing she does seem to be able to redeem her.
Concisely, The Wife of Martin Guerre, does have many themes, but they mostly surface as a result of the binds that tie the characters to their faith, to their families, to each other, and to the things that they strongly believe in. We see how loyalty is weak among the characters, yet, it is an idea strong enough to cause the main problematic situations in the novel.