One of the principle ideas Freud argues in his book is that religion must be understood as an illusion that exists to assist in civilization’s functioning. Such an argument goes against views of religion as sacred teachings provided by God, as Freud’s argument suggests that religion was created by humans to fulfill certain of society’s needs.

For Freud, to call religion an illusion goes beyond arguing that its teachings are merely erroneous or false: “An illusion is not the same thing as an error; nor is it necessarily an error” (30). While Freud believes most illusions are false, he notes that some illusions, like the idea that a prince will marry “a middle-class girl,” are not necessarily “in contradiction with reality” (31). For Freud, whether something is an illusion has to do with its relation to “human wishes.” When false or erroneous ideas exist to fulfill certain human wishes, Freud believes that they can be considered illusions. 

Freud believes that religion can be deemed an illusion due to its role in fulfilling “the oldest, strongest and most urgent wishes of mankind” (30). In Chapter 3, Freud explores how religion emerged in response to early humans' feelings of helplessness.