So we see that we have reason to believe after all in God, freedom, and immortality, although the first Critique led us to think that these would be unknowable. However, although we are required to postulate all of these, and can in fact detect our freedom by detecting the moral law, we cannot cognize them in the sense of attaining a full intellectual understanding. For truly cognizing something requires that we can sense it, and we cannot sense God, freedom, or immortality.
In fact, it is just as well that we cannot sense God, freedom, and immortality. For if we could truly know these things, awe of God, fear of punishment, and desire for reward would become our strongest motivations. We would superficially act as morality demands, but never from the right spirit of pure duty.
The overall argument for the postulates of pure practical reason requires some examination. We need to get from the claim that the object of pure practical reason is the highest good to the claim that we must suppose whatever is necessary to guarantee the highest good in order to follow pure practical reason. There are two problematic things about this transition. The first is seeing which of two senses of the highest good is one with which we started. The second is making sense of the demand for a guarantee of full satisfaction of the highest good to make aiming at it possible.
Kant is not very explicit about why the highest good is the object of pure practical reason. But we can reconstruct his line of reasoning by looking at his talk of dependence on unconditioneds in the first chapter of the Dialectic. For one to aim at some good state of affairs, like getting a paycheck, one must suppose that there is something that makes it good. In this case, what makes this good is perhaps that it provides one with opportunities for amusement and removes the threat of inconveniences, such as eviction or starvation. We then ask what is so good about being amused or not starving. Ultimately, we are driven back to saying that what one aims at actually is the good state in which the worthy person, oneself, is rewarded, and that this aim is dependent on one's worthiness.
Therefore, if we can somehow reach the conclusion that pure practical reason aims at the good, we have an argument that it aims at one particular good, the good of rewarding the worthy. However, this is not quite either of the "highest goods" as Kant describes them. To reward the worthy does not require that the greatest possible rewarding of the worthy take place, nor is it equivalent to merely bringing worthiness, the precondition for any rewarding to take place.
Even so, if we suppose that the object of pure practical reason is the greatest possible good, why would I need a guarantee of full satisfaction of the object of pure practical reason to be motivated by it at all? To see the issue clearly, consider the analogy of a person who desires to be a great painter. The person may take all sorts of steps designed to increase the odds of this, beginning, of course, with doing some painting. Yet no matter how many art lessons that person takes or how long he or she spends contemplating the past greats of painting, there is no guarantee that the person will become a great painter. He or she increases his or her chances, though, via actions, and also stands an overwhelming chance of coming at least partway to his or her goal. And this can be enough for the person's actions to make sense to him or her.