Dr. Pallan Ichaporia

While in one Gathic passage ( Yasna 47.3)  Zarathustra refers to God as the “father” of the Holy Spirit, in another (Yasna 30.3) he describes the Holy and the Fiendish Spirits as “twins.” The conclusion that the Fiendish Spirit, too, was an emanation of Ahura Mazdah’s is unavoidable. But we need not go so far as to assume that Zarathustra imagined the Devil as having directly issued from God. Rather, since free will, too, is a basic tenet of Zarathustrianism, we may think
of the “childbirth” implied in the idea of twinship as having consisted in the emanation by God of undifferentiated “spirit,” which only at the emergence of free will split into two “twin” Spirits of opposite allegiance. Truth alone being, in addition to a primordial principle, also an aspect of Ahura Mazdah, the fact that the Fiendish Spirit had chosen Falsehood would all but obliterate his original connection with God.
It would follow that the names “Holy” and “Fiendish” of the two Spirits did not in the prophet’s view pre-exist the choice which they made, but accrued to either as a result of it.