The obvious theme of Wieland is the criticism of religious fanaticism. The religious fanaticism of both Theodore and his father demonstrates the subjectivity of the human experience. Even more, it suggests that "godliness can corrupt, and absolute godliness can corrupt absolutely". That the horrors that befall the Wieland family come from the direct result of religious enthusiasm indicates Brown's dislike for extreme religious sentiment. Indeed, it is often suggested that Wieland is an attack on Puritanism (though it is also often thought of as a historical allegory, or even one that explores the writing process itself).
Wieland calls into question the sensationalist psychology of the time. The plot is based on the psychological ideology of the time, which was solely based on sensory inputs. While the action is based on this kind of psychology, Brown did not necessarily accept the doctrine without criticism. In fact, he calls into question its validity: the characters are trying to find the truth that is disguised by appearance, and the action – especially Carwin's ventriloquism – shows how difficult it is to find truth simply through sensory evidence. What Brown is concerned with is how the mind can be corrupted by unaccountable and dark impulses.
Ventriloquism exists as a plot device in Wieland, though it goes beyond this simplistic use; Clara Wieland can be thought of as Brown's ventriloquistic voice. Brown, like Carwin, speaks using Clara's voice. It has been suggested that Carwin's confession of his ventriloquism can be equated with Brown's attempt to speak with Clara's voice. When Carwin says, "I exerted all my powers to imitate your voice, your general sentiments, and your language" (Wieland, 240), it can be read that Brown himself has been attempting as an author to speak using a female voice. Seeing ventriloquism as a metaphor in Wieland reaches a deeper truth: that things may not be as they appear, and genuine truth must be actively searched for.