Plenty of leaders of modern-day Western churches believe that every detail of every verse in Genesis is literally true. However, this belief is not universally accepted by Jewish and Christian theologians. The ancient Hebrews did not think the same way that modern-day Western people think, and the book of Genesis is of ancient Hebrew origin. So, modern-day theologians take into consideration the way that the ancient Hebrews interpreted Genesis.
Regarding the proper way to interpret ancient Hebrew writings, Christian theologian William Barclay quotes Old Testament scholar Rev. C.J. Ball as saying the following:
“The Rabbi embodies his lesson in a story, whether parable or allegory or seeming historical narrative; and the last thing he or his disciples would think of is to ask whether the selected persons, events and circumstances which so vividly suggest the doctrine are in themselves real or fictitious. The doctrine is everything; the mode of presentation has no independent value. To make the story the first consideration, and the doctrine it was intended to convey an afterthought as we, with our dry Western literalness, are predisposed to, is to reverse the Jewish order of thinking, and to do unconscious injustice to the authors of many edifying narratives of antiquity.”
Barclay adds, “This is to say that Jewish teachers were more concerned with truth than with fact. They are not interested in the momentary historical events of any story; they are interested only in the eternal truth which the story is designed to illuminate and to convey.”
Jewish theologian Gunther Plaut writes, “The contemporary reader familiar with the history and the nature of the text will have to remember that a literal translation of the Torah may lead to grave misconceptions. Even the ancient Jewish sages, who believed that the Torah was a divinely authored book, did not take the text literally.”
Jewish theologian Nahum Sarna writes, “The literalist approach serves to direct attention to those aspects of the narrative that reflect the time and place of its composition, while it tends to obscure the elements that are meaningful and enduring, thus destroying the biblical message and destroying its relevancy.”
Indeed, modern-day Christian clergy don't insist on a strict literal interpretation of every Old Testament verse. For example, Joshua 10:12-13 states the following:
"At that time Joshua spoke to the LORD in the day when the LORD gave the Amorites over to the sons of Israel, and he said in the sight of Israel, “Sun, stand still at Gibeon, and moon, in the Valley of Aijalon.” And the sun stood still, and the moon stopped, until the nation took vengeance on their enemies. Is this not written in the Book of Jashar? The sun stopped in the midst of heaven and did not hurry to set for about a whole day."
The above verses say that the Sun stood still, reflecting the ancient belief that the Sun revolved around the Earth. We now know that the Earth revolves around the Sun. We understand that the above verses reflect how things appeared to Joshua, and yet we also understand that, if any celestial body stopped moving, then it was the Earth, not the Sun.
So, if a strict literal interpretation is not applied to a Bible verse, then the verse can still be authoritative and teach a spiritual truth. Although the Sun did not literally stand still in Joshua 10:12-13, that passage still tells us that God is in full control of events, and that God used his omnipotence to help the Israelites defeat the Amorites in combat. It didn't matter which celestial object (if any) actually stood still. What mattered is that the God of Israel was the one responsible for the miracle, and the God of Israel was glorified as a result.
Regarding the flood story in the book of Genesis, as German science reporter Werner Keller details in his book The Bible as History, plenty of archaeological evidence exists that ancient Mesopotamia was subjected to extensive flooding, with traces of such flooding reaching all the way to the city of Nineveh.
In 1929, British archaeologist Sir Charles Leonard Woolley was leading an archaeological dig at the ancient Sumerian city of Ur, when Woolley and his workers unexpectedly discovered evidence that the region around Ur had been subjected to a massive flood. Keller writes, “According to Woolley the disaster engulfed an area north-west of the Persian Gulf amounting to 400 miles long and 100 miles wide, looking at the map we should call it today “a local occurrence” – for the inhabitants of the river plains it was however in those days their whole world.”
As seen in Joshua 10:12-13, a story in the Old Testament can contain a detail that is not scientifically accurate because of the perspective of the story-teller, and, yet, the story can still be about an event that actually occurred. Thus, the flood story in Genesis could be about a flood that actually happened even if the perspective of the story-teller was not scientifically accurate. As explained by previously-cited theologians, according to the way that the ancient Hebrews thought, every little detail of a story did not have to be literally true in order for the spiritual lesson of the story to be true.
Today, plenty of Christians do not accept as being literally true the description in Joshua:12-13 of the Sun standing still. Likewise, plenty of Christians do not accept as being literally true the description in Genesis of the entire Earth being covered by a single flood, because empirical data does not give evidence of a global flood.
What all Christians do accept as being literally true are the death, burial and resurrection of Christ Jesus, historical events that took place within a period of three days. Nothing in science does away with those events.
So, one does not have to believe that the entire Earth was literally covered by a flood in order for one to accept the Gospel of Jesus Christ.
Sadly, some naive Christians have been bamboozled by so-called "scientists" who insist that the geological record proves that a global flood took place. Actually, the geological record proves no such thing. Those so-called "scientists" violate the rules of science by insisting that a scientific finding or theory must pass a religious litmus test. Because they insist that the Genesis flood must have been global (because of how they interpret Genesis), they dismiss all geological evidence to the contrary.
Such "scientists" err because they have an ethnocentric understanding of the Bible, which is a common flaw among modern-day Westerners.
Getting modern-day Westerners to be aware of their ethnocentrism is a chore in itself.
 William Barclay, The Mind of Christ (Harper & Row: 1961), p. 79.
 Gunther Plaut, Torah Commentary (Union of American Hebrew Congregations: 1981), p. xx.
 Nahum Sarna, Understanding Genesis (Melton Research Center of the Jewish Theological Seminary of America: 1966), p. 66.
 Werner Keller, The Bible As History (Barnes & Noble Books: 1995), p. 49.
 Ibid., p. 48.