Kant’s concept of religion is recognizing all duties as divine commands. The concept of God employed in religion is an analogical or symbolic concept. Kant’s relation to Christianity was characterized by a tension between Pietism and Enlightenment rationalism. Religion within the Boundaries of Mere Reason aims to test a hypothesis: that there is such a thing as a religion of pure reason and that its relation to revealed (Christian) religion need not be one of conflict but can and should be harmonious. The publication of Kant’s book involved conflict with the Prussian authorities, in which Kant adopted a position of principled obedience while resisting unjust repression and conforming to the rule of law.

Kant holds that the origin of our propensity to evil arises in connection with our unsociable sociability. The effective response to it, therefore, must also be social. We must leave the ethical state of nature and join with others in voluntary ethical community, where our shared ends, conceived as the highest good, under the legislation of a divine lawgiver will promote moral progress among human beings. The existing communities of this kind are churches and ecclesiastical faiths, which fall short of their religious vocation but can and should be reformed so as to live up to it. The relation of rational religion to revealed religion is therefore intended by Kant to be dynamic, with the interpretation of revealed religion enriching rational religion and the reform of revealed religion bringing rational and revealed religion into closer harmony, leading gradually toward the founding of the Kingdom of God on earth.

Justifying grace is for Kant the way religion symbolizes, in terms of our relation to God, our hope to overcome the propensity to evil through the change of heart. Divine forgiveness does not abolish or transcend morality but occurs in accordance with morality. The Son of God symbolizes as vicarious atonement our moral receptivity to God’s mercy. The sacrifice of Jesus Christ on the cross is the way Christianity symbolizes it in revealed religion. For Kant rational religion includes faith in God’s justifying grace. It does not include prevenient or sanctifying grace but does not exclude these either. They are religiously acceptable parts of revealed Christianity, but their reality and our need for them lie beyond what pure reason can know. Some critics claim that Kant’s account of divine grace is inconsistent with itself. But closer examination shows that it is self-consistent, and for Kant rational religion is even consistent with Augustinianism about grace, while neither affirming nor denying it.