Justification of the Postulate of the Immortality of the Soul in Kant's Critique of Practical Reason

Kant justifies the postulate of the immortality of the soul in his Critique of Practical Reason, inasmuch as he demonstrates that without belief in the immortality of the soul, moral law would lose its validity. Since we are determined by moral law, we have the moral intention to foster the supreme good, the belief in the possibility of the supreme good is an implication of such an intention, and if we do not accept the possibility of an infinite progress, we cannot accept the possibility of the supreme good while the acceptance of an infinite progress presupposes the acceptance of the immortality of the soul.