Kant on the soul, free will, and God
In consciousness there is an object (what we are conscious of) and a subject. So it seems we must exist as subjects. But since only objects can be known we can know nothing about ourselves as subjects (our noumenal selves). The mind-body problem therefore cannot be solved. We cannot know whether the mind really is the brain or something else. The brain, known through the senses, and the mind, known by introspection (and philosophical reasoning a priori), certainly do not seem to be the same, but noumenally, i.e. independently of how they are known, the two could be the same. It is impossible for us to know. We can only know things as we know them (of course), and we know things as objects of knowledge, not as subjects. That is to say, when we know something, there is the thing that is known (the object of knowledge) and then there is the knower (the subject). What we know about the subject can only be about the subject considered as an object, we cannot know the subject as subject.
So in a sense we can never know what we really are, but we can have beliefs about this. Kant believes that we are immortal souls, and he justifies this faith by reference to ethics. To live ethically we must have faith in justice. Justice is not done on earth, so we must believe in life after death and justice then. To believe this we must believe that we have immortal souls, that that is what noumenal selves are.
Reason seems to prove various contradictions (“antinomies”) e.g. that the world must, and cannot, have a beginning, or that we do and do not have free-will.
These contradictions arise when we mix up phenomena and noumena. Beginnings belong to space and time, which belong to the objects of our experience. But the world is not an object of experience. It is the collection of all possible experiences. So talk about the beginning of the world is a kind of nonsense (which might be easier to see if you think about "the limits of space" or "the moment when time began").
In nature (i.e. “the world”) all events are causally determined, so in this sense all our actions are determined. But causality might not be a feature of noumena. Ethics requires treating people as free agents responsible for their actions. Anything else is an insult to human dignity. And we cannot help but think of ourselves as free. So faith that really, as noumena, we are free, is both inevitable and justified (but not proved correct).
God is transcendent, i.e. not a possible object of experience, so he is not part of the phenomenal world, not causally active within it. But God could be the cause of the world. Who else would judge our souls? How else can the order and the existence of the world be explained? So faith in God is justified.
Unless we distinguish between phenomena and noumena, we will be led into contradictions. If we do so distinguish, we can see that faith in godless materialism and determinism is not justified (although it might be true). By limiting reason (i.e. by showing what the limits of reason are), we make room for faith.
An interesting essay on Hume and Kant on the rationality of religious belief can be found here: http://www.leaderu.com/truth/3truth08.html