More likely scenario:
The earliest people alive understood and practiced monotheism entirely and worshipped the one true God, and over time as people rebelled and created their own systems of worship and non-worship, they incorporated a lot of those same beliefs into their own belief systems and changed/perverted/or simply forgot the origins. Given that at at least two points in history (creation and flood) all living people believed in the one true God, and that Christ was foretold from the beginning, it is no small wonder that later religions (including the Mithras cult) borrowed heavily. Don't believe everything your high school humanities teacher tells you.
Cobblers. Monotheism didn't rock up in any meaningful way until proto-Judaism. Don't believe anything a fundie tells you - their beliefs were created by bronze age goatherds - you get what you pay for....
Nobody said Christianity is a straight rip from Mithras. Mithras was an Indo-Persian import dating from around the time of the Zoroastrian reformation, thought of to be around 7th to 4th century BC, 600BC being spoken of as very likely. Only in the 1st century CE did the Mithraic cult become particularly popular with the military, almost becoming like a form of freemasonry within the army, but died out one Theodocius had made Christianity the official state religion.
There is little wonder that the worship of Mithra was regarded as a threat to Christianity, especially since it was so widespread. It is not surprising then that Christianity adopted many aspects of the Mithra religion; such as treating Sunday as holy; making 25 December a special day; having a deity born on Earth who looked after man even after ascending to the heavens, where he continued to look down over all, especially his followers, promising them protection in the afterlife. There are many aspects of the Mithra religion that Christianity adopted, but they drew a line at sacrificing bulls in their rites, not surprisingly.
Borrowing from pre-existing has a long tradition in Judaeo Christian scripture, as it is a proven way of drawing in new believers by using well known and loved stories, motifs etc. The Book of Genesis is the most obvious example of this.
Are you saying you believe in the biblical event known as the great flood? If so, why?
Yes. As for "why," in part because:
1. It is recorded in a book that lists so many events and phenomena that can be verified that I have little trouble believing those that cannot be verified.
Go on then...
You don't question anything then?....
2. The occurrence of the flood explains such phenomena as fossils of sea creatures existing at locations thousands of feet above sea level (e.g. the Rocky Mountains), the layering of strata observed at locations such as the Grand Canyon, the petrified forests of Yellowstone, and the vast coal deposits of Kentucky, for example.
Fold mountains and other geological processes... Jeez...
3. The existence of the flood account explains why so many cultures that likely had no exposure to biblical texts later derived flood myths that share commonalities with the Genesis flood account that are beyond coincidence.
The flood account was nicked form the Epic Of Gilgamesh when the hero was called Utnapishtim rather than Noah. The only difference was the change from many gods to one god. During the polytheistic years, the pantheon of gods was seen as a reflection of what was happening on Earth, and so the Gods argued and bickered and had many of the same qualities as humans. Proto-Judaic monotheism changed that - there was now one god, one authority and only room for one opinion. As described above, religions usually having a sense of mission, the best way to convert the great unwashed shambling herd to your new version of Bronze Age woo is to use exactly the same stories and rework them into a new framework. Ditto for tree of knowledge, tree of life, breath of god into man, man created from dust yadda yadda...
In a subsistence society, your main fears are going to be flood, drought, famine, plague... no big surprise that many of the stories are going to contain these themes.
...and for no extra charge.... let's have a look at some other jolly episodes from the good book *cough*
Kill Sons of Sinners
Make ready to slaughter his sons for the guilt of their fathers; Lest they rise and posses the earth, and fill the breadth of the world with tyrants. (Isaiah 14:21 NAB)
God Will Kill Children
The glory of Israel will fly away like a bird, for your children will die at birth or perish in the womb or never even be conceived. Even if your children do survive to grow up, I will take them from you. It will be a terrible day when I turn away and leave you alone. I have watched Israel become as beautiful and pleasant as Tyre. But now Israel will bring out her children to be slaughtered." O LORD, what should I request for your people? I will ask for wombs that don't give birth and breasts that give no milk. The LORD says, "All their wickedness began at Gilgal; there I began to hate them. I will drive them from my land because of their evil actions. I will love them no more because all their leaders are rebels. The people of Israel are stricken. Their roots are dried up; they will bear no more fruit. And if they give birth, I will slaughter their beloved children." (Hosea 9:11-16 NLT)
Kill Men, Women, and Children
"Then I heard the LORD say to the other men, "Follow him through the city and kill everyone whose forehead is not marked. Show no mercy; have no pity! Kill them all – old and young, girls and women and little children. But do not touch anyone with the mark. Begin your task right here at the Temple." So they began by killing the seventy leaders. "Defile the Temple!" the LORD commanded. "Fill its courtyards with the bodies of those you kill! Go!" So they went throughout the city and did as they were told." (Ezekiel 9:5-7 NLT)
God Kills all the First Born of Egypt
And at midnight the LORD killed all the firstborn sons in the land of Egypt, from the firstborn son of Pharaoh, who sat on the throne, to the firstborn son of the captive in the dungeon. Even the firstborn of their livestock were killed. Pharaoh and his officials and all the people of Egypt woke up during the night, and loud wailing was heard throughout the land of Egypt. There was not a single house where someone had not died. (Exodus 12:29-30 NLT)
Kill Old Men and Young Women
"You are my battle-ax and sword," says the LORD. "With you I will shatter nations and destroy many kingdoms. With you I will shatter armies, destroying the horse and rider, the chariot and charioteer. With you I will shatter men and women, old people and children, young men and maidens. With you I will shatter shepherds and flocks, farmers and oxen, captains and rulers. "As you watch, I will repay Babylon and the people of Babylonia for all the wrong they have done to my people in Jerusalem," says the LORD. "Look, O mighty mountain, destroyer of the earth! I am your enemy," says the LORD. "I will raise my fist against you, to roll you down from the heights. When I am finished, you will be nothing but a heap of rubble. You will be desolate forever. Even your stones will never again be used for building. You will be completely wiped out," says the LORD. (Jeremiah 51:20-26)
(Note that after God promises the Israelites a victory against Babylon, the Israelites actually get their butts kicked by them in the next chapter. So much for an all-knowing and all-powerful God.)
God Will Kill the Children of Sinners
If even then you remain hostile toward me and refuse to obey, I will inflict you with seven more disasters for your sins. I will release wild animals that will kill your children and destroy your cattle, so your numbers will dwindle and your roads will be deserted. (Leviticus 26:21-22 NLT)
More Rape and Baby Killing
Anyone who is captured will be run through with a sword. Their little children will be dashed to death right before their eyes. Their homes will be sacked and their wives raped by the attacking hordes. For I will stir up the Medes against Babylon, and no amount of silver or gold will buy them off. The attacking armies will shoot down the young people with arrows. They will have no mercy on helpless babies and will show no compassion for the children. (Isaiah 13:15-18 NLT)
nice one, Yahweh... spread the lurve!