Kant believes natural law is written in the human heart. For him the highest good can be understood as happiness which it is distributed in exact proportion to morality (First Critique). Therefore, in a sense, the highest good can be defined as the combination happiness and virtue. However, this definition is not clearly held in all Kantian writings, something which arises an ambiguity that has been object of an ongoing research. For instance, in the Second Critique Kant also speaks of the supreme good as a consummated or transcended good. A widespread way to approach the issue has been claiming that Kant was merely defined the high good as a two-fold notion: as a transcendent and immanent good, where the first one would deal with the ultimate good, and the second one has to do with a person’s obligations. Other interpretations have been given, but it is not the purpose of this post to discuss it further.
If the highest good is a combination of virtue and happiness, how can a person obtain it for Kant? It should be noted that despite the similarities between Kant and Aristotle, he does not arrive to the same conclusion. Aristotle finds a close relation between the virtuous life and happiness: a moral life leads to happiness. Nonetheless, according to Kant, what is connected to the moral life is instead the worthiness of human beings to deserve or not happiness. If this person obtains happiness is another issue. What makes people worthy of happiness would be their sense of obligation. In this regard, virtuous people would progress toward reaching the highest good, although such a good is not possible in the present natural world. This ultimate good would be a morally better and consummated good, the kingdom of ends, where all ends and moral goals are meet.
The Kantian kingdom of ends is indeed connected to the Christian notion of the kingdom of God. In fact, for Kant the kingdom of ends and the kingdom of God is the same. This kingdom is the ultimate expression of humanity where all its members have the moral law governing over them. The kingdom of nature is also subordinated to the kingdom of ends, so the first one can get meaning from the kingdom of ends. Important to pay attention to this in order to avoid any dualism in Kant’s philosophy. In this sense, from the kingdom of God all-natural motions can receive significant meaning as part of the kingdom of ends. It should be noted that Kant must be read integrally to be coherent because if we read Kant and stay only with the first part of his Critical Reason, we would conclude that Kant is just a mere naturalist philosopher. However, doing a deeper reading with the rest of his philosophical writings, one can observe that Kant does develop perspectives in a broader sense. This, of course, will affect the way the revelation can be useful to his understanding of the primary goal of life (a virtuous humanity as whole). Although Kant never denies the possibility of historical revelation, he claims that philosophical reason is not sufficient to recognize historical faith. For Kant, the will of God must be discerned within the realm of reason and in community.
Kant recognizes that we as human beings struggle with the fact of reaching moral perfection: every person has possibility to act morally but they do not always act according to the moral principles. And humans cannot escape from this continuous struggling because we belong to the kingdom of nature. So in order to overcome the evil of this natural kingdom, we need the kingdom of ends/freedom where a virtuous humanity can be built. For Kant, the kingdom of ends is a necessary ethical community where people help each other and fulfill their duties as moral persons. This is so because moral perfection is a task which concerns everyone and not only to some people. In this respect Kant argues that this ethical community is a means where evil can be overcome via the common good. The human being must leave the ethics of the natural kingdom and becomes a member of the ethical community. Something important to highlight is that for the norms of this ethical community must come from God in order that their members do not exercise power over the another one. Therefore, this would be the maximum ideal which a society can become: it works on the freedom principle and it is immutable in its essence.