A belief was the conviction or acceptance of something by an individual that it existed or was true, especially without proof. Beliefs affected the morals and values of an individual. Beliefs were mostly related to religion. A set of beliefs was known as a belief system.
In 2266, after informing the crew of the USS Enterprise that they would be destroyed, Balok, the commander of the Fesarius, said, "We make assumption you have a deity or deities or some such beliefs which comfort you." (TOS: "The Corbomite Maneuver")
In 3074, Quarren thought that he had known all his life the truth about the USS Voyager without doubt, as depicted in the holoprogram The Voyager Encounter. The Doctor's EMH backup module with his version of history unintentionally threw these beliefs into doubt, but even so The Doctor could not deny what he knew to be true. (VOY: "Living Witness")