Most biblical scholars believe that the biblical Flood story is adapted from an earlier Mesopotamian version. Behind chapters 6-9 of Genesis are Mesopotamian flood traditions, the Atrahasis-myth, and the Gilgamesh-epic (XI). 

Since the nineteenth century, we have known that the authors of Genesis took a pre-existing Mesopotamian narrative of the Flood and its survivor and adapted it to an Israelite conception. The Flood story contains lore from the period known as the Pre-Pottery Neolithic (10 000-6500 BCE). This was a time when the water levels were rising rapidly everywhere due to the melting glaciers after the last ice age. At the same time, people built settlements and domesticated animals. In the biblical flood myth, strands of oral traditions have been combined into a singular story.

The Tower of Babel is an allusion to the early civilizations in Mesopotamia that developed in the 4th millennium BCE. First the Sumerians, then the Akkadians, Amorites, Assyrians, and many other peoples ruled Mesopotamia, building their civilizations on each other's foundations. In short, chapters 2-11 of Genesis portray the ancient Jews' understanding of events in their history in mythic language. 

In Genesis 22, Abraham almost sacrifices his son Isaac to God, but an angel intervenes. Instead, Abraham sacrifices a ram. This story is probably a folktale. and it has parallels in Greek mythology in a story about Nefel and Athamas. Just as King Athamas is about to sacrifice his children, their mother, Nephel sends a ram to fetch Phrixus and save him. 

In another story, Agamemnon's daughter is to be sacrificed to Artemis. A miracle happens, and Ifigeneia is spirited away at the last moment, and a hind takes her place.

In verses Genesis 32:24-30 the patriarch Jacob wrestles with God. This scene is similar to Greek mythology where Menelaus wrestles with the god Proteus. Both have been away for a long time and then wrestle with a god. This is the last obstacle before returning home. In their wrestling match, both of them keep their opponents still for a long time, in order to a kind of test. Each of them has a discussion with god and receive a blessing, in the form of a privileged position and a long life for himself and his family. Jacob becomes a patriarch and Menelaus a hero.

The story of David and Goliath is exaggerated in the style of a legend or even a fairy tale. It is possible that David defeated a great opponent during his career, but like all legends, this story has grown over time. It is possible that the model of the story is the Egyptian Sinuhe. He fights the invincible champion of Retenu, which decides the outcome for the war. In battle, Sinuhe skillfully dodges the enemy’s arrows and shoots the enemy in the neck with his own bow. Sinuhe proceeds to kill the champion with his own weapon. The story is well known from 2000 BCE on.

In the sixth chapter of the Book of Daniel, Daniel is thrown into the lions' den, but they leave him unharmed. A similar legend is told about the prophet Zarathustra. Wild animals do not harm Zarathustra when he is at their mercy. An angel shuts the mouths of the lions surrounding Daniel, just as in the story about Zarathustra, angels shut the mouths of wolves. The Roman legendary figure Androcles was also thrown to wild animals in the arena, but a lion spares him.

The Saga of Samson

Heracles began his work by killing a lion with his bare hands. This is a parallel to Heracles, who in one story found that his arrows, sword and club were useless and so he was forced to fight with his hand against his paw. After closing one entrance to the lion's den with his two mouths, Heracles entered the other unarmed. He wrestled the beast to the ground, wrapped his mighty arms around it and squeezed the lion until it choked. He used the lion's claws to cut off its skin and made a cloak from its invulnerable hide and a helmet from its head.

The Bible says that at one time, ”Samson went and caught three hundred foxes and took sticks of fire and turned them over and put a stick of fire in the middle between the tails. And when he had set fire to the brands, he drove them into the standing grain of the Philistines and burned both the shocks and the standing grain with the vines and olives" (Judg. xv 4-5). The Roman poet Ovid tells of a peasant boy who caught a fox stealing his chickens, tied straw to its tail and set it on fire. The fox escaped into the neighbouring fields, set them on fire and destroyed the village's entire harvest (Fasti, Book 4, lines 681–712).

Samson, like many Greek heroes, has weaknesses that contribute to his downfall. His weakness for women, particularly his relationship with Delilah, leads to his capture and eventual death. This is reminiscent of the tragic weaknesses often found in Greek heroes, such as the hubris of Achilles or the cunning of Odysseus. Heracles visited the city of Thespiae, where he met a prostitute named Daphne. He was so taken with her that he stayed with her for a year and neglected his work.

Cleomedes, is a Greek hero from Astypalaea. During the Olympic Games, He went mad when he was not declared the winner and killed his opponent, Iccus of Epidaurus. He returned to his homeland and toppled the pillar supporting the roof of a school, killing about sixty children. Similarly, Samson is blinded, captured and brings down the central pillars of the building, causing it to fall on the Philistines and himself. Samson seizes the gateposts of the city of Gaza and carries them on his shoulders up a mountain. In his final act, Samson thus sacrifices himself to destroy his enemies, similar to Greek heroes who often sacrifice themselves for the common good.

There are books in the Old Testament that should be considered essentially historical. But they are by no means objective descriptions of what actually happened. The Deuteronomistic history, the books from Deuteronomy to 2. Kings, is an interpretation of the disasters of 722 and 586 BCE, that made it possible to accept the loss of Israel without abandoning Yahweh. The loss of Israel was not to be taken as evidence that Yahweh was not the highest God or that he had abandoned his people. Israel's loss meant the opposite: Yahweh was a universal God who could even use other nations to judge his own people. These judgments were clear to all: Israel had been condemned, while Judah had taken a different path. Yahweh rewarded keeping the covenant law with Judah's political continuation.

Yes, there is a historical core in the Old Testament. All stories in it are not pure fiction. However, it is a known fact among scholars that these stories are blended with fiction. The Old Testament includes myth, saga, legends, ethiologies, history, annals, reports, anecdotes, and novella. (Watson E. Mills, Roger Aubrey Bullard - Mercer dictionary of the Bible – Genre in the Old Testament, page 323).

Professor George Coats names the principal narrative genres in the Old Testament as saga, tale, novella, legend, history, report, fable, etiology and myth. (Genesis, 1983, 5-10).

"There's been a great deal of work done on the Bible and its relationship to comparative folklore and mythology. The general underlying assumption is that there's nothing in the Bible that can't be found in some form - or to which some analogy cannot be found - in some mythology or folklore elsewhere. " (Northrop Frye and Jay Macpherson: Biblical and Classical Myths – The Mythological Framework of Western Culture, University of Toronto Press, 2004, page 42)

So to answer the question at the beginning of the page: enough to convince a believer that the book is valid historically. And not nearly enough to convince anyone else. For anyone else, archaeological evidence never gets off the ground (pun intended). How do we know that the God depicted in these stories existed anywhere else but inside the minds of human beings? 

There is no reason to assume so. The Abrahamic God has given no evidence of himself that could withstand critical scrutiny; no advanced medical, scientific or cultural knowledge or knowledge of future events. Only unverifiable stories of miracles.