Spenta Mainyu

Enter Spenta Mainyu:As long as the simple Zoroastrian believed in God, Hormazd, and his adversary the evil one, Ahriman, things went without spenta mainyu. The more learned said that it was an appellation of Hormazd. And long before them, in the good old days of the Vendidad, Ahura Mazda, the most spenta mainyu, had anghra mainyu as his opponent. According to the Zurvanites, who were perhaps as old as the Achaemenians in the 6th century BC, and as young as the authors of Bundahishn and Vichitakiha-i Zatsparam in the 9th century AD, good and evil were twins begotten by Zurvân Akarna, Boundless Time. So the simple Zoroastrian was, more or less, following tradition.

But with the advent of Zoroastrian studies, led and encouraged by Western scholars, a change set in. Studies of the Gathas and the later Avesta revealed that spenta mainyu was referred to as an entity. And since then, almost all Zoroastrians and those who are well acquainted with the Zarathushtrian religion know the term spenta mainyu. Because the Gathas and the later Avesta were translated into English and other European languages, mostly by Christian scholars who had the Holy Ghost or Holy Spirit in mind, the term has conventionally come to mean the Holy Spirit. The general notion about it is that it has an adversary, Anghra Mainyu, the evil spirit. The two are locked in a pitched life-and-death combat. The victory, of course, will go to the Holy Spirit.

http://www.zoroastrian.org.uk/vohuman/Article/Spenta%20Mainyu.htm