According to Religion and the Individual, many psychopathologists think so (p. 259). It quotes three at some length:
1. Sigmund Freud: “It has repeatedly been pointed out (by myself and in particular by Theodor Reik) in how great detail the analogy between religion and obsessional neurosis can be followed out, and how many of the peculiarities and vicissitudes in the formation of religion can be understood in that light. And it tallies well with this that devout believers are safeguarded in a high degree against the risk of certain neurotic illnesses; their acceptance of the universal neurosis spares them the task of constructing a personal one.” - The Future of an Illusion (1927)
2. James Dittes: “The psychological research reflects an overwhelming consensus that religion (at least as measured by the research, usually institutional affiliation or adherence to conservative traditional doctrines) is associated with awareness of personal inadequacies, either generally or in response to particular crisis or threat situations; with objective evidence of inadequacy, such as low intelligence; with strong responsiveness to the suggestions of persons or other external influences; and with an array of what may be called desperate and generally unadaptive defensive maneuvers.” -Psychology of Religion (1969)
3. Albert Ellis: “Devout, orthodox, or dogmatic religion (or what we might call religiosity) is significantly correlated with emotional disturbance. People largely disturb themselves by believing in absolutistic shoulds, oughts, and musts…. The devoutly religious person tends to be inflexibly closed, intolerant, and unchanging. Religiosity, therefore, is in many respects equivalent to irrational thinking and emotional disturbance.” - Psychotherapy and Atheistic Values (1980)
These views are shared by numerous others, including Erich Fromm (Psychoanalysis and Religion, 1950). And they’re “reflected in diagnostic instruments like the Minnesota Multiphasic Personality Inventory (MMPI), which include items that ask about religious belief, prayer, and experiences of the presence of God, and treat affirmative answers as evidence of psychopathology (p. 259).”
Is religion a form of mental illness? Again, it seems that the answer depends on how one defines “mental illness.” At the very least, it seems safe to say that religion is associated with some very troubling feelings and ways of thinking; that religious people have little reason to consider themselves superior to non-religious people; and that much more research into the precise nature (and possible pathology) of religious belief is needed.
Alas, the likelihood of much research being conducted seems small. Although U.S. religious groups take in more than $60 billion every year, little if any of that money is used to fund objective evaluations of the nature, basis, and practical effects of the religious beliefs at their core. The political firestorms which often accompany such diverse subjects as stem cell research, the development of new and better abortion/contraception techniques, the teaching of evolution, gay rights, and court decisions which demand a strict separation of church and state merely give a hint of the firestorm which would erupt if anyone in the U.S. government stood up and publicly suggested that millions of dollars ought to be funneled to university psychologists attempting to answer the question “Is Christianity a form of insanity?”
Until such questions are openly posed, adequately funded, and honestly answered, we must get by the best we can with the flickering light offered by such research as has already been done.
Although that research is hardly conclusive, there is little in it that theists can take comfort in….