Kant says there are three kinds of religious delusions, all of which we should avoid. We should not believe in miracles, since we do not have direct, scientific evidence of miracles occurring today or in the days of old. Kant also speaks against religious mysteries, since, like miracles, their existence cannot be proven "through reason" (6:194). Finally, we should not believe that religious rituals or professions of faith will make us more righteous in God's eyes. There is nothing wrong with participating in the ritual of religion; in fact, Kant says that prayer, church attendance, initiation rituals, and communion can sustain us in our "true service to God" (6:193). But we should not mistake participation in these practices for true moral conduct.
Kant says that our inability to know God's will limits our ability to make moral judgments. Typically, people deem religious doctrines good or bad after undergoing a religious revelation that supposedly shows them the doctrine's worth. But Kant points out that we have no legitimate, tangible evidence that religious revelations are real, so we must avoid using them to condone or condemn religious doctrines.
In Christianity, "grace" is specifically defined as the healing forgiveness and blessing granted by God to deserving humans. Kant believes that humans must not sit around waiting for God to rain grace on their heads, excusing their bad behavior by saying that forgiveness lies in God's hands. However, he does believe in the concept of grace to a limited extent. He thinks that humans must do all they can to behave morally, and then hope that God will bless them by granting grace. He says, "whoever does, in a disposition of true devotion to duty, as much as lies within his power to satisfy his obligation can legitimately hope that what lies outside his power will be supplemented by the supreme wisdom in some way or another" (6:171).
In this passage, Kant suggests that grace can only be given after we have made a concerted effort to meet our moral obligations. Grace can absolve an imperfect individual from sin, but only after she has done everything within her power to become a good person. Kant also seems to assert that this grace will simply absolve us of old sins, rather than help make us better people. He says we "indeed have no rightful claim" to a kind of grace that absolves of us all sin, past, present, and future (6:75). Kant also warns us against getting too smug about our victories over sin, because even as we become better people the ultimate test of our moral fortitude is our actual conduct, not our past successes (6:77). Kant seem to think that human beings are not transformed into perfect moral creatures when they receive justifying grace. Rather, grace makes us realize that our past victories are less important than our diligent efforts to become better people.
Kant does not insist that grace exists. We have no proof that it does, and Kant says we can only believe in things of which we have tangible evidence. He suggests that we should hope grace does exist, without counting on its existence. He says grace is just "the idea of an improved disposition of which God alone has cognition" (6:76). Believing in grace will help us become better people, for we must be able to envision ourselves gradually moving toward moral perfection. Believing in grace will also comfort those people who hold themselves to rigorous standards. A moral "human being will pronounce a stern judgment upon himself, for he cannot bribe his reason" (6:77). This person can take comfort in the idea that if she works hard to be good, God might forgive those mistakes that she cannot forgive in herself.