Kant’s newly acquired interest in Zarathushtra was indeed so great that in 1802 he contemplated including that name in the title of his work. But, as we should have expected, the sources of his interest are hardly those of Nietzsche. Two of the titles thus suggested by Kant are:
1) Zoroaster: oder die Philosophie im Ganzen ihres Inbegriffs unter einem Prinzip zusammengefasst;
2) Zoroaster: das Ideal der physisch und zugleich moralisch praktischen Vernunft in Einem Sinnen-Objekt vereinigt.
In one set of passages Kant maintains that the religious interpretation of all duties as divine commands is not supplementary, later interpretation, but is, for every moral being, immediately and necessarily given together with the apprehension of the duties, i. e. the categorical imperative leads directly to God, and affords surety of His reality.
In a second set of passages, Kant makes no reference to the existence of God but only to the Idea of God. But in these passages also, duties are alleged to be apprehensible only as divine commands.
In yet another set of passages Kant suggests that God Himself, and not merely the Idea of God as a trans-subjective Being, is immanent in the human spirit:
God is not a being outside me, but merely a thought in me. God is the morally practical self-legislative Reason. Therefore only a God in me, about me, and over me. 
The proposition: There is a God says nothing more than: There is in the human morally self-determining Reason a highest principle which determines itself, and finds itself compelled unremittingly to act in accordance with such a principle. [Loc. Cit.]
There is a God, namely, in the Idea of the morally practical Reason which [determines] itself to a continuous oversight as well as guidance of the actions according to one principle, like to a Zoroaster. 
Kant’s reading of the Zend-Avesta, and also his reading of Lichtenberg’s eulogies of Spinoza, are here evidence:
Similarly to the Zoroastrian principle of intuiting all things in God, and of dictating how they should be (like Lichtenberg) and the capacity of thought as inner intuition to develop out of itself. [609-10]
Many of passages are directed against the view of God as a substance:
Cosmotheology. It is an object of the morally practical Reason, which contains the principle of all human duties as being divine commands, and yet dos not require us to assume a special substance existing outside man. 
There is a being (Wesen) in me, which though distinct from me stands to me in relations of causal efficacy, and which, itself free, i.e. not dependant upon the law of nature in space and time, inwardly directs me (justifies or condemns), and I, as man, am myself this being. It is not a substance outside me; and what is strangest of all, the causality is a determination to action in freedom, and not as a necessity of nature. [824-25]
God must be represented not as substance outside me, but as [the] highest moral principle in me. But indirectly as a power in me (gods do not exist) [it] is the Ideal of power and wisdom in one concept [Ahura Mazdâ]. 
The Idea (not concept) of God is not the concept of a substance. The personality which we ascribe to it, which is also bound up with the singleness of its object (not a plurality of gods)… 
Clearly Kant’s views have undergone considerable change since the writing of the Critique of Practical Reason.
Erich Adickes, Kants Opus Postumum, dargestellt und beurteilt, Berlin, 1920.
Vaihinger, Die Philosophie des als ob, 1913, 721-.
Normann Kemp Smith, A Commentary to Kant’s Critique Pure Reason, Macmillan & Co., 1930.
Eckart Förster, Kant: The Enlightenment Zoroastrian
December 7, 1994, Guest Lecturer to Eckart Förster, Stanford Un.
October 29, 1994, The Second World Gathic Conference, Costa Mesa.