Q: Why is it important that the Bible be taken as truth? Would the Christian faith be diminished if the Bible were metaphorical, or is there some other issue with such an interpretation?
The Bible says it is the Word of God. The Christian faith has been diminished because the Bible in many circles, including so-called Christian circles, stopped being regarded as history and metaphorical meanings have been attached to many passages. The Bible is history. It declares itself to be written by the Holy Spirit [one of the manifestations of the Godhead] as he moved on men and women of God. It was not dictated by God. The Koran makes the claim that an angel told Mohammed what to write for example. Instead it begins at the beginning of creation and declares what God did step by step from the dawn of history working through individuals then one specific family that of a man called Abraham. He became a people through his son Isaac called today the jews. He chose a specific family of the jews notably the line of king David [the second king of Israel] and through this line eventually God came to this planet in the form of a man.
The Bible foretold and documents all of this. There are literally hundreds of prophecies about the Messiah outlining in great detail when he would come the circumstances of his birth and death, and what he would do through His people. The Bible is a historical record of this and much more. The Old Testament concerns the nations of the world and their interaction with Israel and prophecies concerning many of them with dead on accuracy. The New Testament is the record of the the life and teaching, miracles, death, burial and resurrection of the Messiah [Yeshua or Jesus in Greek] told by eye witnesses [the 4 Gospels], and the rest is early history of the spread of the followers of Jesus through letters of instruction and encouragement to the first jews and gentiles [non-jews=Greek speakers] to believe in Him by Saul of Tarsus [the apostle Paul] and some of the other original 12 disciples of Jesus. They are all first hand accounts of events that occurred from approximately 30 to 90 AD.
This is not to say that the Bible does not have passages with metaphorical content. The Psalms were written as poetry, and there are wonderful passages of great literary merit in its pages. The 66 books that make up the cannon of Scripture have no myths or legends or fantastical tales. Once we decide that this or that is really just metaphorical or a myth we become relativists and where does it stop.
A) If the Bible were not true, I would be in a mess. If Jesus hadnt risen, I would have no hope for the future and for eternity; I would not be forgiven; I would not know a God who loves the world and who is benevolently involved in the nitty gritty or peoples lives.
B) It is important that the Bible be taken as literal truth because it is literal truth. I suspect that would be the same answer that one would give if asked why it is important that scientific discoveries be taken as literally true.
I want to address another question I think is embedded in your initial question: Why do you take the Bible as literally true? The historicity of both the Old and New Testament has been established by scholars over the centuries. The Old Testament has been confirmed by hard evidence and documentary evidence uncovered by archaeology and by the accuracy of the copying process.
The New Testament emerged from 24,970 partial and complete manuscripts found in 15 languages. Iliad, by comparison, was fashioned from 643 partial and complete manuscripts. There are also internal and external evidences for the reliability of the New Testament. External evidences involve both Christian and non-Christian sources. Moreover, the writers of the New Testament wrote as eyewitnesses or from firsthand information.
There is too much information to fully do this topic justice, but Josh McDowells book, The New Evidence That Demands a Verdict, Chapters 3 and 4, lays out the evidence for the historic reliability of both testaments.
Of course this only addresses the parts of Scripture that are history. The prophetic parts of Scripture are easily verifiable. Did what the prophets say come true? The Bible contains 1,000 prophesies; about half have been fulfilled in detail; the other half concern the end times. Again, there is too much information to condense, but if you are interested check out Mark Hitchcocks book, The Amazing Claims of Biblical Prophecy.
Would the Christian faith be diminished if the Bible were metaphorical?
I doubt there would be a “Christian faith” if the Bible were metaphorical. Different metaphorical interpretations abound, but they are like nailing jelly to the wall. They look good for the moment, but they do not withstand the test of time. Besides, no one attempts to make metaphors out historical or scientific facts.
For example, the Nazi regime, murdered 20 million people (not counting soldiers). Would any number of metaphors in any way clarify, enhance or explain the horrific event?