What evidence is there to support the position that the Bible is truly the Word of God to mankind? Christianity generally accepts the Bible to be the Word of God, and the Bible itself makes this claim (i.e. 2 Peter 1:16-21).


  16 For we did not follow cleverly devised myths when we made known to
  you the power and coming of our Lord Jesus Christ, but we were
  eyewitnesses of his majesty. 17 For when he received honor and glory
  from God the Father, and the voice was borne to him by the Majestic
  Glory, “This is my beloved Son,[i] with whom I am well pleased,” 18 we
  ourselves heard this very voice borne from heaven, for we were with
  him on the holy mountain. 19 And we have the prophetic word more fully
  confirmed, to which you will do well to pay attention as to a lamp
  shining in a dark place, until the day dawns and the morning star
  rises in your hearts, 20 knowing this first of all, that no prophecy
  of Scripture comes from someone's own interpretation. 21 For no
  prophecy was ever produced by the will of man, but men spoke from God
  as they were carried along by the Holy Spirit.


Yet, just because the Bible claims to be a revelation from God does not make it so, as other holy books of other religions claim to be the same.

What, then, are the primary lines of evidence that demonstrate that the Bible is, indeed, the Word of God to mankind (outside of its own claims)?
First, like @Harmen, I'd like to clarify what I mean when I say the Bible is God's word.  From there, I can get to a better answer to your question.
What does the statement &quot;The Bible is God's Word&quot; mean?
The traditional Evangelical view in the Scripture as God's word is that it was inspired by God.  The actual term used is &quot;God-Breathed&quot;.  Further clarification on this statement is taken from truthnet.org.

The Bible was written under “Inspiration” from the Holy Spirit.  The
word inspire is dervived from the latin word “inspirare” meaning  “To
breathe upon or into something”.  God revealed Himself through
individuals to speak to a much larger audience.  In the same way some
one would pick up a pen.  The pen would becomes the tool in the hand
of the writer, the prophet is God’s pen in revelation.
The Bible is a collection of books.  As the prophets received
revelation the books were colleceted in manuscript1 form.  The
orginal manuscript, written by the prophet is known as the autograph.
The autograph is inspired by God from the first generation.  The
autograph is then copied to additional manuscripts.  As the
manuscripts wear out over time from use and enviroment the manuscript
is again copied onto a new scroll. This process of copying the old
manuscript  to a new manuscript is known as manuscript transmission.
Over a period of time the manuscripts from accepted prophets were
collected.  This collection of manuscripts is known to us as the
Bible.


What evidence exists to support the statement that the Bible is God's Word?
In the book, Evidence That Demands a Verdict, Josh Mc Dowell notes that if God created man with a desire to know Him, we would expect His message to have some unique properties:

It would be widely distributed so man could attain it easily
It would be preserved through time without corruption
It would be completely accurate historically.
It would not be prone to scientific error or false beliefs held by the people of that time.
It would present true, unified answers to the difficult questions of life.

The Bible stands alone as the only religious text that can claim it meets all the above criteria.
Test 1:
The first qualification listed above is any message from the Creator would have to distribute His message to a wide audience. The Bible is next to none in this test. It is the most published book in history, with the widest distribution of any published work. It has been translated into more languages than any other book. It is the most sold book in history. It was the first book published with moveable type. It is still the #1 best seller of any book. Now, none of these feats prove that the Bible was inspired. They are, however, consistent with what we would expect of God's message if He were trying to let us know about Himself and His plan for us. In other words, one cannot disqualify the Bible on this point. Many other ancient writings fall short, but it behaves as we would expect.
Test 2:
The Bible has orders of magnitude more manuscript evidence, supporting the fact that it has been preserved though history, than any other book in antiquity. More can be found here.
Test 3:
Extra-Biblical accounts of history and supporting archaeological discovers confirm a great deal of what's in the Bible.  More can be found here.
Test 4:
Of course, there's little agreement on one of the big issues - Young-Earth Creation vs. Old-Earth Creation vs. Evolution, so I won't even both trying to use that.  The evidence that exists can be used to support any view if you twist it enough.  However, aside from that, the Bible does outline medical wisdom that is far ahead of it's time.
Again, from here:

When Genesis was written, The Greeks were beginning to tell of
Apollos' flight across the sky in a flaming chariot. The Egyptians
were worshipping the sun as Ra, deifying it. The Mesopotamians
referred to the sun as &quot;Shamosh&quot; and called it the god of justice.
Genesis, however, calls the sun &quot;a light in the expanse of the
heavens&quot; and views it as a thing, one created by God. That the Bible
does not follow the naiveté of those ancient religions is often
overlooked, since modern man is much more knowledgeable in the
mechanics of nature. We take for granted that someone touching an
infectious person or a corpse should practice good hygiene and wash
thoroughly in running water before proceeding to anything else, but
this &quot;discovery&quot; has only been a medical reality for 150 years. The
book of Leviticus, though, requires this same procedure.
One cannot
find ideas as arcane as blood-letting or consuming ram's horn for
fertility, or all the other mythical cures for ills that were thought
to be science in those days. The Bible is not a science book. It does
not focus on scientific facts about the creation, but where it
mentions those things, it is accurate in its representation. This is
exactly what we'd expect if the Bible had its origin in the One who
created the universe and its scientific laws.

There's also a neat little chart showing Biblical teachings compared to the &quot;science&quot; of the time here.
Test 5, from the same source as above:

Thus far we have examined several evidences of the accuracy and the
reliability of the Bible by comparing it to outside sources and what
we know is true. Now, I'd like to turn our attention to the text of
the Bible itself to show how it validates itself as the word of God.
Again, I remind you that we have a collection of different documents
that were written over one and a half millennium, that are devoted to
discussing the most controversial and emotionally charged topics man
has known. The incredible thing is that they all agree. Taken
together, the Bible presents a single, unified message of actions and
attitudes by which man can live. This is an unprecedented feat.
To have sixty six books written by about forty authors, from kings and
nobles to fishermen and soldiers, in three languages and on three
continents, be of the same mind is just not humanly possible. Why, the
editorial writers in our newspapers can't even agree when they come
from the same culture and similar educational backgrounds.
To demonstrate the remarkability of this accomplishment, we can
propose an experiment. Imagine a classroom of thirty students at the
high-school level. The teacher has decided on the class writing a
novel for a class project. Each student will be assigned one chapter
and they will then gather the papers together to assemble the finished
work. The topic chosen is &quot;Why God is important in man's life,&quot; but
there is no outline and there are no rules as to what that statement
means. Because the students are all the same age and live in the same
area at the same point in time, they have a tremendous advantage over
the Biblical writers, but to expect a congruent work is ridiculous.
The fact that the Bible is a unified message shows that its origin
comes from beyond man.
Because the Bible claims it is the word of God, it requires of itself
a stricter assessment. The Old Testament is filled with the
authoritative phrase &quot;Thus sayeth the LORD&quot; . The fact that men
recognized it from the time it was first penned as authoritative gives
it a measure of strength. The laws that were required of the Jews were
very arduous. Because they chose to accept them as commandments from
God before any significant length of time had elapsed to mythicalize
them shows that the people believed with their lives that these
documents were from God. Jesus Himself validates the Old Testament by
regarding it as the word of God and authoritative in all things

Also, not mentioned in the tests from Josh McDowell, there's the test of prophecy.
The Bible contains many prophecies, some made hundreds of years before they were fulfilled.  Many of these are very specific, not vague statements like the ones you see from Nostradamus, for example.
A few, with probability calculations are presented here.

Finally, none of this constitutes absolute proof, and atheists and non-Judeo-Christians have been arguing against all of this evidence for years.  The idea that there would be people that scoff at the Biblical account of creation and the flood, for example, is even predicted in 2 Peter 3.

2 Peter 3:3-7 (KJV)
3 Knowing this first, that there shall come in the last days scoffers, walking after their own lusts, 4 And saying,
Where is the promise of his coming? for since the fathers fell asleep,
all things continue as they were from the beginning of the creation.
5 For this they willingly are ignorant of, that by the word of God the
heavens were of old, and the earth standing out of the water and in
the water: 6 Whereby the world that then was, being overflowed with
water, perished: 7 But the heavens and the earth, which are now, by the
same word are kept in store, reserved unto fire against the day of
judgment and perdition of ungodly men.

This, of course, is perfectly in harmony with Jesus' teaching in the book of John 3 (Also KJV).

16 For God so loved the world, that he gave his only begotten Son,
that whosoever believeth in him should not perish, but have
everlasting life. 17 For God sent not his Son into the world to
condemn the world; but that the world through him might be saved. 18
He that believeth on him is not condemned: but he that believeth not
is condemned already, because he hath not believed in the name of the
only begotten Son of God. 19 And this is the condemnation, that light
is come into the world, and men loved darkness rather than light,
because their deeds were evil. 20 For every one that doeth evil hateth
the light, neither cometh to the light, lest his deeds should be
reproved. 21 But he that doeth truth cometh to the light, that his
deeds may be made manifest, that they are wrought in God.

In short, no amount of evidence is going to be enough to prove the Bible's truth, that it's God's Word, or even the existence of God to those that simply don't want to believe.  The evidence is there, it's up to each of us to determine whether we're convinced by it, and how we want to respond to it.