Zoroastrianism is based on the teachings of Zoroaster, a 6th century Iranian prophet and philosopher. Zoroastrianism is almost identical with Mazdaism (the worship of Ahura Mazda, the supreme deity exalted by Zoroaster). Zoroastrianism survives today in isolated areas of the Middle East, primarily Iran, but more prosperously in India, where the descendants of Zoroastrian Persian immigrants are known as Parsis, or Parsees. In India the religion is called Parsiism. As with all false religions, Zoroastrianism is incompatible with Christianity.
For one thing, the claim that Zoroastrianism is perhaps the oldest monotheistic religion and that it had an influence on Judaism, Christianity, or Islam is simply not true. While Zoroastrianism is credited to having its origins in the 6th century BC, it only enters recorded history in the 5th century BC. This is in contrast to the Bible, where most historians and scholars put the writing of the Pentateuch (Genesis-Deuteronomy written by Moses) between 1446–1406 BC during Israel’s wanderings in the desert. This means the Old Testament pre-dates the Avesta (official religious text of Zoroastrianism) by close to 900 years.
While it is not surprising to see antagonists of Christianity on the internet and in books try to discredit Christianity by claiming the Zoroastrian influence on it, it is clear that the concept of one God and the need for a Savior was established much earlier by the Hebrew people. Even the prophet Isaiah spoke of the virgin birth of Christ and recorded it around 701 BC, which still precedes Zoroastrianism by 100 years.
Secondly, Zoroastrianism states that active participation in life through good thoughts, good words, and good deeds is necessary to ensure happiness and to keep chaos at bay. There are various religious rituals that must be observed and a variety of acts and deeds to be performed to ensure salvation. This is in stark contrast to Christianity which teaches that Christ is the only way to salvation (John 14:6) and that our salvation cannot be earned (Ephesians 2:8–9). This is the unique difference between Christianity and all false religions. In Christ, salvation is a free gift from God apart from works which save no one (Romans 3:20, 28). All other religions require works of some kind to appease God. Thankfully, our Bible is crystal clear on salvation, how to receive it, what is true, and what is not.
The prophet Zoroaster supposedly received what he recorded in the Avesta from a vision of Vohu Manah (moral enlightenment, possibly an angel of sorts) while drawing water from the Daiti river. Zoroaster is the sole author. This method of “enlightenment” is similar to the prophet Mohammed of Islam receiving a vision from the angel Gabriel, and passing it down for about three centuries by word of mouth before being recorded by scribes in the Qu’ran. Still, the source is only one man, and a person should question the accuracy of the recitations over a long period such as that.
Compare these “revelations” with Bible: 40 authors of 66 books over a span of 1600 years (55 generations), most of whom were not acquainted with one another, and who came from different backgrounds (judge, prophet, king, priest, shepherd, scribe, soldier, fisherman, physician), different locations (tents, palaces, dungeons, cities, deserts) and written under different circumstances in three different languages (Hebrew, Greek, Aramaic) who ALL convey the same common theme about God, and whose writings are accepted as the inerrant, true, and inspired word of God recorded through men (2 Timothy 3:16). The accuracy of the original texts has been verified over and over each time an ancient biblical manuscript has been discovered, the Dead Sea Scrolls being among the most recent.
It is abundantly clear that Zoroastrianism is yet another religion where salvation is works-based. There is no evidence of any divine influence in the religious writings, and it is clearly not impacting the world and changing lives today the way our relevant, living, all-powerful God is doing. The Bible, which could not possibly exist and claim the things it does unless it truly IS the inspired Word of God, has the power to change lives on a massive scale. From a Christian perspective, we hope and pray that the few who still do follow the teachings of Zoroaster would be exposed to the truth of Christ and realize that the salvation they seek to achieve by their good deeds is actually a free gift through Him.
 Got Questions Ministries. (2010). Got Questions? Bible Questions Answered. Bellingham, WA: Logos Bible Software.