On the Schleiermacher episode, we spent some time comparing On Religion to Kant's religious arguments, particularly citing Kant's Critique of Practical Reason. Kant did not try to prove God's existence or the soul's immortality. Rather, he postulated those concepts as helpful ways to help realize the summum bonum, the highest good. "Postulate" is defined as a "a hypothesis advanced as an essential presupposition, condition, or premise of a train of reasoning."
With that in mind, read along as you listen to this passage from the Critique of Practical Reason. Reviewing it may help highlight what Schleiermacher was rejecting:
The Immortality of the Soul as a Postulate of Pure Practical Reason
The realization of the summum bonum [highest good] in the world is the necessary object of a will determinable by the moral law. But in this will the perfect accordance of the mind with the moral law is the supreme condition of the summum bonum. This then must be possible, as well as its object, since it is contained in the command to promote the latter.
Now, the perfect accordance of the will with the moral law is holiness, a perfection of which no rational being of the sensible world is capable at any moment of his existence. Since, nevertheless, it is required as practically necessary, it can only be found in a progress in infinitum towards that perfect accordance, and on the principles of pure practical reason it is necessary to assume such a practical progress as the real object of our will.
Now, this endless progress is only possible on the supposition of an endless duration of the existence and personality of the same rational being (which is called the immortality of the soul). The summum bonum, then, practically is only possible on the supposition of the immortality of the soul.
Consequently this immortality, being inseparably connected with the moral law, is a postulate of pure practical reason. (By which I mean a theoretical proposition, not demonstrable as such, but which is an inseparable result of an unconditional a priori practical law).
This principle of the moral destination of our nature, namely, that it is only in an endless progress that we can attain perfect accordance with the moral law, is of the greatest use, not merely for the present purpose of supplementing the impotence of speculative reason, but also with respect to religion. In default of it, either the moral law is quite degraded from its holiness, being made out to be indulgent and conformable to our convenience, or else men strain their notions of their vocation and their expectation to an unattainable goal, hoping to acquire complete holiness of will. And so they lose themselves in fanatical theosophic dreams, which wholly contradict self-knowledge.
In both cases the unceasing effort to obey punctually and thoroughly a strict and inflexible command of reason, which yet is not ideal but real, is only hindered. For a rational but finite being, the only thing possible is an endless progress from the lower to higher degrees of moral perfection. The Infinite Being, to whom the condition of time is nothing, sees in this to us endless succession a whole of accordance with the moral law. And the holiness which his command inexorably requires, in order to be true to his justice in the share which He assigns to each in the summum bonum, is to be found in a single intellectual intuition of the whole existence of rational beings.
All that can be expected of the creature in respect of the hope of this participation would be the consciousness of his tried character, by which from the progress he has hitherto made from the worse to the morally better, and the immutability of purpose which has thus become known to him, he may hope for a further unbroken continuance of the same, however long his existence may last, even beyond this life.*
And thus he may hope, not indeed here, nor in any imaginable point of his future existence, but only in the endlessness of his duration (which God alone can survey) to be perfectly adequate to his will (without indulgence or excuse, which do not harmonize with justice).
* It seems, nevertheless, impossible for a creature to have the conviction of his unwavering firmness of mind in the progress towards goodness. On this account the Christian religion makes it come only from the same Spirit that works sanctification, that is, this firm purpose, and with it the consciousness of steadfastness in the moral progress.
But naturally one who is conscious that he has persevered through a long portion of his life up to the end in the progress to the better, and this genuine moral motives, may well have the comforting hope, though not the certainty, that even in an existence prolonged beyond this life he will continue in these principles.
And although he is never justified here in his own eyes, nor can ever hope to be so in the increased perfection of his nature, to which he looks forward, together with an increase of duties, nevertheless in this progress which, though it is directed to a goal infinitely remote, yet is in God's sight regarded as equivalent to possession, he may have a prospect of a blessed future.
For this is the word that reason employs to designate perfect well-being independent of all contingent causes of the world, and which, like holiness, is an idea that can be contained only in an endless progress and its totality, and consequently is never fully attained by a creature.