What would Christianity be without 1. one God 2. an arch nemesis to this one God 3. angels and fallen angels 4. the Kingdom of Heaven 5. Heaven and Hell in the afterlife contingent upon one's good and bad deeds in this life 6. the Messiah 7. a Ressurection 8. a Judgement day?
Could Christian religious institution even function without these concepts?
1. That's essential to Christianity, without which it would be pagan.
2. Christianity doesn't need an arch-nemesis.
3. I think it would still work. I consider the earthly life more important than the heavenly.
4. It would be more of a social movement then, without the promise of an alternate reality. I consider the kingdom of heaven an important concept. I had no idea this existed in Zoroastrianism.
5. Similar to my response to point 4.
6. I'm not interested in a messiah concept. There is no point believing in a messiah if you are not willing to be an agent of change socially, politically and economically, fighting for justice, working to minimise economic foot-print, giving to charity, blessing the meek, being humble, etc.
7, 8. They're part of my ideology.
I personally do not believe Christianity "hangs" on Zoroastrianism because it is a hybrid of at least two other thought systems, particularly Judaism and Hellenism. As long as Christianity is the syncretism of at least two thought systems, it will continue to exist. You can cut out the Zoroastrianism and Christianity will still be Christianity because it doesn't absolutely need Zoroastrianism to continue to be Christianity.
Christianity only ceases to be what it is when you cut out all thought systems except for one. For example, you may eliminate the other two, leaving yourself with Hellenism. When you're left with just one thought system, there's no syncretism so Christianity ceases to be.
Christianity doesn't absolutely need Zoroastrianism to exist, but it does absolutely need syncretism to exist.
I for one am not particularly concerned about the continued existence of Christianity, as long as it fulfills its mission. When we have fulfilled our duty, our tradition will no longer be important. Christianity is a temporary religion, not a permanent one. (So let us all rally ourselves for the cause and be sacrificed as cannon fodder.)
Christianity assimilated Zoroastrianism and Hellenism. These two thought systems have since disappeared and ceased to exist as major world religions. When Christianity has absorbed, assimilated and co-opted all other pagan traditions, it will itself cease to exist. It will surrender its sovereignty to the last remaining and permanent Abrahamic and monotheistic tradition. This will be either Judaism or Islam. (I'm putting my bet on Judaism.)
When Christianity has fulfilled its mission, Judaism and Islam can do whatever they like with Christianity.