The most obvious theme in the collection of personal essays is the outline of one's beliefs or principles in life. Each essay—regardless of the person's background and characteristics—is written with conviction. The concept of one's individual belief system is contrasted and interwoven with the social norms that the world around them dictates. After all, one's beliefs are constructed, mostly, from one's firsthand experiences in life. However, these personal beliefs can also be created or manipulated by the larger belief systems existing in the outside world (e.g., religion, political ideologies, secondhand knowledge, the media, advertisements, pop culture, etc.).

In most of the essays in the collection, there is more or less a push-pull dynamic between keeping one's personal beliefs intact as the beliefs of others try to contradict the former. The participants in the "This I Believe" series come from an eclectic range of backgrounds, nationalities, political stances, faiths, and cultures. This makes their personal beliefs more pronounced to the readers because their perspectives and ideas may not fit in with the principles of others. This superimposition of the individual against the society (represented by the readers) makes the collection of essays compelling.

Although the essayists in the collection come from different cultural backgrounds, they are, for the most part, residents of the United States. This makes the collection of essays—although personal and microscopic in perspective individually—seem like a large mosaic of perspectives representing the diversity of America. One essay might offer a belief or perspective that differs from the next piece, and this contradiction is representative of America's dynamic population.

Each essay writer is identified, partly, by his or her career. One is a lawyer, while another essayist is a member of the parole board. One's job and what it entails is essential to the person's story and belief system. For instance, an essayist who has worked within the penal system for decades has a nuanced perspective on prison reform and civil rights. Another essayist's personal beliefs are shaped by their job as a hospital clerk.