The postulates are not knowledge claims — Kant is not arguing that we can prove the existence of God or the soul's immortality by theoretical reasoning. That way is barred by the Critique of Pure Reason. Instead, he argues that morality requires us to act as though these propositions were true, because without them the demands of practical reason could not be coherently sustained.
Freedom is postulated because moral obligation presupposes that we can act otherwise — determinism would make praise and blame incoherent. Immortality is postulated because the moral law demands holiness, a perfect alignment of will with duty that no finite being can achieve in a single lifetime. God is postulated as the guarantor of the 'highest good': the necessary connection between virtue and the happiness proportionate to it.
Kant introduces the idea of 'moral faith' — a rational, practically grounded belief in the postulates. This is neither superstition nor mere hope; it is the attitude that morality itself demands of us. A rational being who takes the moral law seriously must live in the light of these commitments, even though they exceed any possible experience.
The postulates are developed in the 'Dialectic of Pure Practical Reason,' especially the chapters on the Highest Good and the Immortality of the Soul.
