Religion within the Boundaries of Mere Reason (Religion, hereafter) is a passionate statement of Kant's mature philosophy of religion. As the title suggests, Kant believes that religious experience is best understood through rationalism, an important philosophical movement in the 18th, 19th, and 20th centuries that argues we know some things intuitively, not through experience, and that we can determine certain absolute truths by relying on this intuitive knowledge.
In Religion, Kant explores the legitimacy of religious experience. He argues that organized religion often gets in the way of genuine religious experience, thereby threatening the moral development of humanity. This argument spans four sections.
In Part One, Kant discusses whether human nature is inherently evil or inherently good. He thinks we have a predisposition to engage in good behavior, which comes in three instinctual urges: propagating the species, fostering meaningful, stable relationships with others, and respecting the moral law. Kant thinks that in addition to our inclination to be good, we have a simultaneous propensity for evil or immoral behavior. Kant suggests that we will see the truth of his thesis if we examine the evil abroad in the world around us. The state of current political and social life will convince skeptics that people are in need of moral development.
In Part Two, Kant argues that it is possible for us to become morally good by following the example of Jesus Christ, who resisted enticing temptations, and by instituting a wholehearted change in behavior.
In Part Three, Kant says it may be possible to create a society that fosters moral behavior. Such a society would emulate the ideal "church invisible," an association of individuals committed to living morally upright lives. Kant says that rituals and professions of faith are not essential for the establishment of a morally sound religious community. We can know our duty to observe the moral law without the aid of miracles or common religious practices.
In Part Four, Kant continues to criticize certain aspects of organized religion. He says that much of existing organized religion does not help people improve their moral standing. Incantations, professions of faith, and even consistent participation in religious services cannot transform the morally corrupt into the morally upright.