Mental health problems represent a significant and growing challenge across the globe. Although progress has been made developing effective therapeutic interventions, this paper argues that a lack of metaphysical diversity hinders scientific advancement in this crucial area. Using the United States as an example, demographic data is reported illustrating the under-representation of theists in psychology and social work, the two largest providers of mental health services in America. Drawing from McIntosh’s theory of privilege, it is posited that secular perspectives enjoy an advantaged status in mental health discourse, a status that is reinforced by the dissemination of secular narratives in culture-shaping forums. To illustrate how limited diversity impacts service provision, cognitive-behavioral therapy (CBT) is used as a case example. While the secular values embedded in CBT suggest it is often a good fit with secular clients, these same values may limit its validity with committed theists, such as Muslims. The paper concludes by suggesting that increased metaphysical diversity will help advance scientific knowledge by fostering the creation of research agendas that reflect the values of the diverse demographic groups mental health professionals are called to serve.