Peck postulates that there are four stages of human spiritual development:^[15][16]

• Stage I is chaotic, disordered, and reckless. Very young children are in Stage I. They may defy and disobey and are unwilling to accept a will greater than their own. They are egoistical and lack empathy for others. Criminals are often people who have never grown out of Stage I.
• Stage II is the stage at which a person has blind faith in authority figures and sees the world as divided simply into good and evil, right and wrong, us and them. Once children learn to obey their parents and other authority figures (often out of fear or shame), they reach Stage II. Many religious people are Stage II. With blind faith comes humility and a willingness to obey and serve. The majority of conventionally moralistic, law-abiding citizens never move out of Stage II.
• Stage III is the stage of scientific skepticism and questioning. A Stage III person does not accept claims based on faith, but is only convinced with logic. Many people working in scientific and technological research are in Stage III. Often they reject the existence of spiritual or supernatural forces, since these are difficult to measure or prove scientifically. Those who do retain their spiritual beliefs move away from the simple, official doctrines of fundamentalism.
• Stage IV is the stage at which an individual enjoys the mystery and beauty of nature and existence. While retaining skepticism, s/he starts perceiving grand patterns in nature and develops a deeper understanding of good and evil, forgiveness and mercy, compassion and love. His/her religiousness and spirituality differ from that of a Stage II person, in the sense that s/he does not accept things through blind faith or out of fear, but from genuine belief. S/he does not judge people harshly or seek to inflict punishment on them for their transgressions. This is the stage of loving others as yourself, losing your attachment to your ego, and forgiving your enemies. Stage IV people are labeled  mystics.

Peck argues that while transitions from Stage I to Stage II are sharp, transitions from Stage III to Stage IV are gradual. Nonetheless, these changes are noticeable and mark a significant difference in the personality of the individual.