HKB
possible only on condition of the existence of God, it inseparably connects the supposition of this with duty; that is, it is morally necessary to assume the existence of God.
It must be remarked here that this moral necessity is subjective, that is, it is a want, and not objective, that is, itself a duty, for there cannot be a duty to suppose the existence of anything (since this concerns only the theoretical employment of reason). Moreover, it is not meant by this that it is necessary to suppose the existence of God as a basis of all obligation in general (for this rests, as has been sufficiently proved, simply on the autonomy of reason itself). What belongs to duty here is only the endeavour to realize and promote the summum bonum in the world, the possibility of which can therefore be postulated; and as our reason finds it not conceivable except on the supposition of a supreme intelligence, the admission of this existence is therefore connected with the consciousness of our duty, although the admission itself belongs to the domain of speculative reason. Considered in respect of this alone, as a principle of explanation, it may be called a hypothesis, but in reference to the intelligibility of an object given us by the moral law (the summum bonum), and consequently of a requirement for practical purposes, it may be called faith, that is to say a pure rational faith, since pure reason (both in its theoretical and practical use) is the sole source from which it springs.
{BOOK2|CHAPTER2 ^paragraph 45}