Robert Cloninger, who has worked extensively in the areas of personality disorder, alcoholism, and “positive” mental health, has some interesting things to say about this.
He arranges the different levels of organization and integration of the human personality, in order of complexity, in a quasi-“evolutionary” sequence.
In Cloninger’s schema, personality can be broken down into temperament (biological predispositions to react a certain way), character (learned patterns of behaviour), motivation (our will to perform an act consciously) and psyche (thought and “higher-order” cognitive functions). He and his co-workers have delineated five levels at which the psyche “develops”: the lowest is “recognition of memory and facts” (stage 1), and the highest are “awareness of higher truths” (stage 4) and “the intuitive sense of goodness” (stage 5).
According to him, a failure to progress through stage 4 can lead to agnosticism and atheism. Thus, while an agnostic or atheist does not have a “personality disorder” in the full sense of the term, he has not developed the two highest stages of personality, which are more or less unique to humans. Most of us never fully make it through stage 5; those who do are the “great minds”, “enlightened ones”, or - as we Catholics would put it - saints.
While this schema is based on secular science and evolutionary theory to some extent, it does posit a basic difference in complexity and organization between a “theist” (who has made it past stage 4 and is struggling with stage 5) and an “atheist” (who is struggling, or has given up the struggle, to cross stage 4.)
Does this answer the question?