Full Idea
Cyrus the Great was a Zoroastrian, believing in one, eternal, beneficent being, 'Creator of all things through the holy spirit'.
Gist of Idea
Zoroastrians believed in one eternal beneficent being, Creator through the holy spirit
Source
Paul Johnson (The History of the Jews [1987], Pt II)
Book Reference
Johnson,Paul: 'A History of the Jews' [Phoenix 1993], p.85
A Reaction
Is this the actual origin of monotheism, or did they absorb this idea from the Jews? The interesting bit is the fact that the supreme being (called Marduk) is 'beneficent', which one doesn't associate with these remote and supposed pagans.