I don't know if religious folks could or should be locked up. That wouldn't be so feasible. But the question of mental health overlaps with the very philosophical question of the meaning or purpose of life. Should our species be so theistic and religious? Is that what's best for us? According to what standard? Would it be better if we were more rational and skeptical? Would the world be a better place? The answers aren't obvious, but the point is that psychiatrists have occupational trouble addressing such questions.