One reason is the manuscript evidence for the Scriptures. Check out this video: http://www.youtube.com/watch?v
We also believe the Bible because:
1) God in His Word claims to be its author.
2) The evidence within Scripture confirms the claim that the Bible is God’s Word.
3) There is external evidence in archaelogy and history that confirm the truthfulness of the Bible. So, we believe the Bible because it is historically accurate. Jesus Christ really is who he said he is (the eternal, divine Son of God) and he really did what he said he did; namely, he lived a perfect life of obedience to God’s law, died on the cross, rose on the 3rd day, appeared to over 500 witnesses (1 Corinthians 15:6), ascended to the right hand of the Father, and will come again to judge the living and the dead.
4) The testimony of God’s Holy Spirit in our hearts.
For further reading on this subject, check out chapter 1, “The Bible: The Basis of Our Confession” in Confessing Christ by Calvin Knox Cummings.