The Avesta is the source scripture for Zorastrian ideology (Malandra, 2005). It is a collation of a small fraction of "a variety of written and oral traditions" collated by Sasanian priests during the Sasanian period, but originating in sources "almost entirely non-contemporaneous with times and eras that one wants to understand through them" (Malandra, 2005).
"The Avesta is the great achievement of learned Zoroastrian priests who collected, edited, and codified a variety of written and oral traditions during the Sasanian period, that is, during an era far removed from the times when the constituent pieces of the tradition were composed. Those constituent pieces that have survived to today, however, represent only a fraction of what the Sasanian priests produced." (Malandra, 2005). In other words, the ideas
"Zoroastrianism is the oldest of the revealed world-religions, and it has probably had more influence on mankind, directly and indirectly, than any other single faith" (Boyce, 1979: pp. 1).
"Zoroaster was thus the first to teach the doctrines of an individual judgment, Heaven and Hell, the future resurrection of the body, the general Last Judgment, and life everlasting for the reunited soul and body. These doctrines were to become familiar articles of faith to much of mankind, through borrowings by Judaism, Christianity and Islam; yet it is in Zoroastrianism itself that they have their fullest logical coherence....” - (Boyce, 1979: pp. 29)
Malandra, W. W. (2005). Zoroastrianism i. Historial Review up to the Arab Conquest. Retrieved from http://www.iranicaonline.org/articles/zoroastrianism-i-historical-review