The Apostle Paul compares Jesus to Adam in the book of Romans, chapter 5, verses 12-21, and in such a fashion that if Adam were not a real man, one would have to conclude that Christ were not either.
In Luke 3, the "beloved physician" Luke provides a Genealogy of Jesus including his step-father Joseph, King David, his father Jesse, Boaz (husband of Ruth), Judah, Jacob, Isaac, Abraham, Shem, Noah, Seth, and (yes) Adam. Again, if Adam is only metaphorical, if Adam was nonexistent, if Adam was not a real person, what are we to think of all the other people in this genealogical line? Are they also metaphorical or nonexistent?
One of the primary purposes of providing such genealogies is to tie the people and events therein to history. A similar genealogy is provided in Genesis 5, from Adam to Noah, making it clear that not only did Luke want his book to be taken as history, but the author of Genesis desired that his book be understood in the same manner -- as a historical narrative.
The Apostle Peter (in 1 Peter 3:19-20 and 2 Peter 2:5) refers to the flood of Noah, which we are told about in Genesis 7 and 8, as an actual historical event.
Abraham, known primarily from Genesis 12-23, is referred to over 70 times in the New Testament as a real person, to whom God made irrevocable promises, from whom the Israelites were physically descended, and after whom our faith is to be modeled.
Abraham, Isaac and Jacob are even referred to by Jesus Himself as living people in Matthew 22:32, Mark 12:26-27, and Luke 20:37-38.
Genesis was clearly intended to be taken by its author as history, and it was clearly taken to be such by both the Israelites and the apostles, and further, by Jesus Himself.
For the Christian, the reality, historical reliability, and veracity of Genesis is absolutely beyond question.
Many Christians today seek to compromise the message of God with modern theories like Darwinian Evolution or the Big Bang. However, there is no evidence whatsoever for Darwinian Evolution (and thus no reason to even consider supporting it, let alone rejecting Christ or His word over it); and Big Bang theory itself, since it proclaims the beginning of time, demands an Eternal Uncaused First Cause, Self Directed and Self Motivated and therefore Personal, not bound by time and therefore Unchanging, in charge of time and all things temporal at the beginning and therefore still in charge of time and all things temporal and therefore Omnipotent and Omniscient...
In other words, the Big Bang can only be true if the God of scripture exists. Yet, in an interesting paradox, if the God of scripture exists, the Big Bang cannot be completely accurate, given God's own clear account of His creation in the book of Genesis.
Thus we see that when the Christian regards the evidence in its proper context, internal and external, Genesis must be taken literally because of the impossibility of the contrary.