The Old Testament books Genesis and Exodus detail the origins of humanity and the earliest events in
ancient Israelite history. 4 out of 10 Americans not only believe the books are 100% factual, but they also consider them to be foundational to their religion.  However, when reviewing the evidence, the claims made by the Bible's ancient authors become highly suspect.
Adam and Eve
To those in many denominations of the Christian faith, the story of Adam and Eve is central to understanding the crucifixion of Jesus, the specialness of humanity, and the origin of evil on Earth.  Yet, most genetic scientists believe it would be nearly impossible for humanity to have originated from 2 people. As Dennis Venema, a biologist at Trinity Western University, explained: "That would be against all the genomic evidence that we've assembled over the last 20 years." This evidence suggests humans evolved from a group of several thousand individuals who lived about 150,000 years ago.
The Great Flood
Given that Adam and Eve are foundational to certain denominations of Christianity, many Christians believe it is essential for all events in the book of Genesis to have occurred.  One of the most significant and global reaching events is the flood described in the story of Noah's Ark. Creationist Christians use a form of pseudoscience referred to as "Flood Geology" to explain geological phenomena, such as the Grand Canyon and fossil remains, not only to "prove" that the flood happened, but also to "prove" the earth is only 6,000 years old. The scientific consensus, however, is that there is not only a lack of evidence for the biblical flood, but Creationist explanations of geological phenomena go completely against both the evidence and laws of physics. It is interesting to note that the Epic of Gilgamesh, an ancient Sumerian story written prior to 2,000 BCE, also contains a flood myth very similar to the one written in the Bible. Given the dating of both stories (Noah's Flood was written at about 800 BCE) it is quite plausible that the Biblical account originated from the Sumerian one, though it is also likely both stories came from the same tradition. In fact, this tradition could be based on an actual regional flood that scientists believe may have occurred around 5,000 BCE.
In the story of Exodus, Moses led 600,000 ancient Hebrews out of a life of slavery in Egypt to the "Promised Land" in the region of Canaan (modern day Palestine.) Once there, the book of Joshua details the mass slaughter of the local inhabitants to make room for the tribes of Israel. This narrative formed the foundation of Jewish identity for thousands of years, and reinforced the idea they were God's chosen people. However, archaeological evidence suggests none of it ever happened. After a century of research by archaeologists and Egyptologists, there is simply no evidence of such a large population of Hebrews either living in Egypt or traversing the desert on the way to Canaan. In addition, ancient pottery shards and cult objects discovered during archaeological digs in Palestine show that there was no disruptive influx of other cultures during the time of the supposed Exodus. On top of that, Canaanite settlements lacking pig bones (a common indicator for ancient Jewish populations) have been dated prior to the Exodus, thus indicating the Jews originated from the area. As archaeologist Ze'ev Herzog pointed out "The Israelites never were in Egypt. They never came from abroad. This whole chain is broken. It is not a historical one. It is a later legendary reconstruction—made in the seventh century [BCE]—of a history that never happened."
The Ten Commandments
Some Christians believe that the Ten Commandments are the foundation of much of western legal systems and democracy. They believe the commandments were so special and unique; they must have come from God. However, the Egyptian Book of the Dead (1800 BCE) had a list of statements very similar to many of the Ten Commandments:
"I have done away sin for thee and not acted fraudulently or deceitfully.
I have not belittled God.
I have not inflicted pain or caused another to weep.
I have not murdered or given such an order.
I have not used false balances or scales.
I have not purloined (held back) the offerings to the gods.
I have not stolen.
I have not uttered lies or curses."
Since scholars believe the Ten Commandments were written in about 1500 BCE, it is possible the ancient Israelites used the Book of the Dead as a template. However, even if they didn't, the sheer similarity indicates that the Ten Commandments weren't particularly unique.
For Jews in particular, circumcision is a big deal. In Genesis 17, Abraham promised the god Yahweh all of his descendents would be circumcised. Many people today believe that this practice originated with the Jews at that specific moment detailed in the Bible. Yet, the evidence actually suggests that ancient Egyptians may have embraced the practice prior to the ancient Hebrews. In fact, they've found mummies from as far back as 4000 BCE that had their foreskin removed.
Many adherents of Abrahamic religions consider the earliest books in the Old Testament to be essential for understanding the nature of humanity and our relationship with God. Yet, thanks to genetics, biology, geology, Egyptology and archaeology, we now know none of it ever happened. Not only that, but much of the stories and traditions were likely borrowed from Israel's closest neighbors.
Good Adam and Eve NPR Article:
Interfaith site comparing Noah's Flood to Epic of Gilgamesh:
Good Christian Article on the Historicity of Exodus:
Good History of Circumcision: