The Knick features Sister Harriet, a complex character whose same-sex relationship is portrayed with dignity. While she faces tragic consequences due to early 20th-century societal prejudice, the show's narrative critiques this intolerance rather than her identity, affirming the worth of her life and love despite external obstacles.

The show features Jacob Speight, a patient who expresses gender dysphoria and seeks a sex change. Dr. Thackery performs an orchiectomy, reflecting the era's limited medical understanding. The portrayal is factual to the historical period, neither affirming modern trans identities nor actively denigrating the character, but rather depicting the challenges and primitive medical responses to such cases in the early 20th century.

The series frequently portrays Christian institutions and many of its adherents as hypocritical, rigid, or ineffective in the face of societal and medical challenges. While individual characters may exhibit genuine faith, the narrative often critiques the restrictive nature of religious doctrine and the moral failings of those who claim piety.

Judaism is depicted with respect, primarily through the Chickering family and Dr. Levi Zinberg, who are portrayed as intelligent, ethical, and contributing members of society. The series acknowledges the prejudice faced by Jewish individuals but frames their faith and cultural identity positively.