"How can we trust a book that was written so long ago?"
This is a fairly common question, and a very valid one to ask. There are many reasons we can trust the Bible today. I'm going to give you 6 reasons to trust the Bible.
First, a little intro. The first reason I'm about to list is here to set the stage for the other 5 reasons. The other 5 reasons are all independent of each other, but all relate back to the first reason. (This will be more clear by the end of the first reason).
Reason #1: Self-proclamation
All scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness.
2 Timothy 3:16
Now certainly, I wouldn't expect anyone to trust the Bible solely for this reason alone. However, this self-proclamation does have its importance. If we had the other 5 reasons, but we didn't have self-proclamation, we could still be left wondering if this is really from God. The self-proclamation of 2 Timothy 3:16 eliminates any sort of in-between area. Either the Bible is the Word of God, or it is not the Word of God. Now for some reasons to trust this self-proclamation.
Reason #2: Unity
The Bible is not just one book, written by one or two people. The word Bible comes from the greek word biblia, which means collection of books. That's what the Bible is, a collection of 66 books. The Bible has over 40 different authors, coming from all walks of life. For instance, Moses was brought up under the Egyptian teachings, Peter was a fisherman, Luke was a doctor, and Matthew was a tax collector. The Bible was also written in 3 different languages, across 3 different continents, spanning a period of over 1,500 years. There was no telephone back then, no internet, and all the various authors could not have possibly gotten together and colluded to write the Bible (given the time span).
Yet, despite all of these facts, there is unprecedented unity when it comes to any issue in the Bible. For instance, what you find in one book of the Bible about giving your cares and anxieties to God can also be found in other books of the Bible. Furthermore, there are no contradictions. This sort of unity could only be attained by having one guiding author (God). This guiding author would have had to be around for over 1,500 years. Therefore, no man could be the guiding author of the Bible.
Reason #3: Transforming Ability
For the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and spirit, of joints and marrow, and discerning the thoughts and intentions of the heart.
There have been many cases in which people have broken addictions with the Bible, hence the "word of God is living and active." People have overcome addictions to pornography, smoking, drugs, alcohol, etc. in the name of Christ. This is done by applying the word of God to your life (again, living and active). Here's a couple of instances of verses that support this transforming ability.
So then brothers, we are debtors, not to the flesh, to live according to the flesh.
What this passage is saying is that we are not obligated to our addictions. This passage is proclaiming that we have control over our addictions, and that they should not have control over us (we are debtors, not to the flesh).
Let not your hearts be troubled. Believe in God; believe also in me.
Humble yourselfs, therefore, under the mighty hand of God so that at the proper time he may exalt you, casting all your anxieties on him, because he cares for you.
1 Peter 5:6-7
These 2 passages are about not worrying, and giving your cares and anxieties to God. In return, God fills you with his peace. Applying these passages (and there's many others like it) to your life can give you peace, even in troubling times.
We could go on to list passages that support the transforming ability all day (and I haven't even quoted from Psalms or Proverbs yet). In fact, there are many Christian books dedicated to applying the word of God to your life.
Reason #4: Archeology
This is another topic to which entire books can be dedicated to. I'll just give a few examples of archeology that supports the Bible. Feel free to research these examples of archeology.
Then the Lord rained on Sodom and Gomorrah sulfur and fire from the Lord out of heaven.
Other translations may say brimstone (which is sulfur). Archeologists have discovered the remains of Sodom and Gomorrah, and have found the purest form of sulfur ever found in nature. This discovery supports the historical event recorded in the Bible. Speaking of historical events supported by archeology, the reamins of Pharaoh's chariot army have been discovered, scattered across the bottom of the Red Sea. These chariot pieces, horse bones, and human bones support the events of the Exodus.
There are many historical accounts from the times of the Roman Empire that support the life of Jesus Christ. These accounts not only support that Jesus was a real person that walked the earth, but also that Jesus died by crucifixion on the cross, and that Jesus was the founder of Christianity.
Reason #5: Bible Prophecy
The book of Zechariah was written in approximately 520 BC, centuries before Jesus walked the earth. In the book of Zechariah, we see this passage, which is Zechariah having a vision of a sign-act prophecy.
Then I said to them, "If it seems good to you, give me my wages; but if not, keep them." And they weighed out as my wages thirty pieces of silver. Then the Lord said to me, "Throw it to the potter" - the lordly price at which I was priced by them. So I took the thirty pieces of silver and threw them into the house of the Lord, to the potter.
Now in this vision Zechariah is acting out the event of the future. And in the New Testament, we see this prophecy being fulfilled. Jesus was sold for thirty pieces of silver, and this money was thrown back into the temple (aka the house of the Lord). The chief priests said they could not put the money into the treasury, since it was blood money. So they used the money to buy the potter's field (Matthew 27: 3-8).
So, you have the thirty pieces of silver, thrown into the house of the Lord, and given to the potter, and these specific things were written down centuries before it was fulfilled. This is certainly not the only example. The Old Testament is filled with hundreds of prophecies about Jesus, including his birthplace and death on the cross.
There's Bible prophecy this is being fulfilled before our very eyes. The book of Revelation describes a one-world government and one-world currency. This world is heading straight in that direction, it's Bible prophecy being fulfilled.
Reason #6: Divine Knowledge
Here I'm talking about knowledge that could not be known under normal conditions. Yet, this knowledge must have had a source, a divine source. Let's go through a few examples.
The burden of the word of the Lord concerning Israel: Thus declares the Lord, who stretched out the heavens and founded the earth and formed the spirit of man within him:
Recall that Zechariah was written in 520 BC. Notice how Zechariah is describing the Lord, "who stretched out the heavens." Modern science has shown that the universe is expanding, and the technology to verify that was not around at the time Zechariah was written. However, this passage accurately indicates that the universe is expanding (the hebrew word for heavens in this passage is referring to outer space, which would be the universe). How would Zechariah have known to write something like that, when the universe expanding could not be verified scientifically in those days? But yet what Zechariah wrote is accurate regarding the expansion of the universe. Zechariah must have received divine knowledge.
It's been shown that the ideal ratio for building a ship is to have the length be six times as long as its width. In Genesis 6:15, Noah was given the dimensions for the ark, 300 cubits for the length, and 50 cubits for the width. 300 = 6 x 50. Noah's ark matched the ideal shipbuilding ratio by having the length six times as long as the width. Noah didn't discover this ratio on his own by a long process of guessing and testing, he was given divine instructions for building the ark.
There are modern day examples of people gaining divine knowledge. For instance, in the book, Heaven is for Real, a 3-year-old boy takes a trip to heaven during a life threatening surgery. While he was in heaven, the boy learned about the activites his dad and his dad's grandfather used to do together. Perhaps even more amazing is how the young boy learned about his second sister, who died before he was born when his mother had a miscarriage. When the boy came out of the surgery and came back from heaven, he began repeating these things to his parents. How could a three-year-old know about his mother's miscarriage if he had not been to heaven and received divine knowledge? A mother doesn't tell a three-year-old about having a miscarriage.
There are many, MANY reasons you can trust the Bible today. The things presented here only scratch the surface on some of these topics.