There are several important themes in A Handmaid's Tale. These include the following:
The objectification of women: women and women's bodies become property in Gilead, and Handmaids lose any personal agency their sexuality.
Religious totalitarianism: Gilead uses state power to enforce religious laws and to control access to fertile women. Gilead's regimented society is the product of religious zealotry that confuses control with morality.
Complicity: many of the characters in the book, including Offred, are complicit in maintaining in the Gilead regime. A recurring theme is how, in the face of the power of the state, people will do anything to avoid punishment.
The male gaze: this novel shows how women are subjugated by the way men look at them, or that being seen can be a form of violence. This is exemplified by the secret police, know as "the eyes," and is a deliberate invocation of the well-being one supposedly feels under the protection of an all-seeing God. In Gilead, however, this protection turns into a threat.
Taken together, these themes all contribute to a central message, which is that political control of women's bodies and their ability to reproduce is morally wrong. Gilead's totalitarianism is a direct result of an explicitly patriarchal system that uses religious authority to erase female agency. While Atwood's story is fictional, another message is that it would be easy to slip into a Gilead-like society unless we are vigilant about defending personal agency.