What is the theme in The Sirens of Titan by Kurt Vonnegut?

Organized Religion is the main theme of the story. Vonnegut makes some pointed suggestions about organized religion throughout the novel. In the very first sentences of introduction, the narrator speaks of the time in which the story takes place as the "Nightmare Ages" before people had learned to find the meaning of life. This age was filled with "gimcrack religions", the narrator says, calling them "big business" (p. 7). This period is placed as the period between World War II and the fictional "Third Great Depression". In other words, the time the narrator describes could be the time contemporary to Vonnegut's 1959 novel.