ZOROASTRIANISM is one of the oldest and was, at one time, one of the most widely practiced religions of the ancient world.
Zoroastrianism is probably the first, if not one of the first religions touting belief in one God (Ahura Mazda).
"In Zoroastrianism, Ahura Mazda is the beginning and the end [Alpha and Omega], the creator of everything that can and cannot be seen, the Eternal, the Pure and the only Truth. --Wikipedia
Zarathustra is the original name of this prophet, however, due to the Greeks' difficulty pronouncing it, they changed his name to Zoroaster. (Nope, he was not a creation of Friedrich Nietzsche, Stanley Kubrick or Richard Strauss.)
Bible scholar P.R. Ackroyd states: "the whole eschatological scheme...of the Last Judgment, rewards and punishments, etc., within which immortality is achieved, is manifestly Zoroastrian in origin and inspiration." See: "Also Sprach Zarathustra"
What else do they believe?
When Zoroaster was 30 he went into the water of a river and emerged with a vision that was later refined into a belief in one God and his adversary (Angra Mainyu). His vision included that all people exist to learn the difference between good and evil with the primary goal of overcoming evil.
Zoroaster developed the concepts of freewill, heaven and hell, as well as a personal and final judgment after death (determined by a person's decision to follow good or evil). He also instituted the ideas of resurrection, and, ultimately, the destruction and renovation of the world.
Zoroastrians believe a Savior will come before the end of the world who will be born of a virgin impregnated by the seed of Zoroaster.
"IT IS BELIEVED THAT KEY CONCEPTS OF ZOROASTRIAN ESCHATOLOGY AND DEMONOLOGY HAVE HAD INFLUENCE ON THE ABRAHAMIC RELIGIONS." --Wikipedia.
"Abrahamic religions" includes Christianity. And there are many more commonalities between this "first" monotheistic belief system and Christianity...
For example, Zarathustra has been compared to Jesus more than any other religious figure in history. They both were against the established religions of their times and, as a result, were met with anger and contempt. They also were both considered revolutionary figures about whom very little is known.