Typically a main message in a novel is referred to as a theme. Most novels have enough layers so that there are multiple theme on various topics. Topics are different than themes. A topic is a subject touched upon in a novel and a theme is a stance or commentary on a theme.
In The Giverthere are several themes or big messages to choose from. One of the main messages revolves around the importance of individuals to have free will and have an identity apart from the group to which they belong.
Jonas' society in the novel promotes a one size fits all approach to life. People are expected to hit important milestones at the same age, respond in unison in certain situations, and each family is even the same size. This challenges the idea of individuality and free will which Jonas eventually challenges.
The control of Jonas' society and threat of violence or being released keeps the individuals subdued and complacent throughout the novel. This paranoia caused by the society keeps the individuals afraid to rebel or even to have many unique and personal thoughts. This mob thinking is dangerous in its lack of understanding and inhuman style.