I knew Christ before I really knew the bible, so I suppose I have to give Christ credit for helping me know it was the truth.
When I got a little older, and started hearing the nay-sayers, I noticed that the nay-sayers were not Christians, and they weren't even good secular people. The people I respected, respected God's word.
Then, when I was a teenager I heard the things that agnostics will say to discredit God's word, I went to Dr. Ralph Smith, former president of Southwestern Baptist Theological Seminary. I explained to him that I knew God's word was real, but that I didn't know how to answer the taunts of athiests, and I knew that there must BE an answer. He explained to me that no answer will really satisfy a non-Christian, but agreed there ARE answers, and with great patience, he gave me my first theology lessons on the floor of his private study.
He showed me that what some called contradictions are in fact not. For instance - Genesis and creation are not contradictions - one is a summary, another is a detailed account. The same story, with different emphasis on different areas.
By the time I was 16, I understood that God's word could survive any test man could throw at it. I once started tracing Christ's lineage backwards, using scriptures. When I hit a "snag", Dr. Smith smiled, and showed me how a name changed here, and then there, all recorded in scripture.
When I wanted to do a "time line" of historical events - Dr. Smith reminded me that calendars change, and helped me create an accurate time line that compared the calendars. Once I understood that - he told me to "rebuild" the biblical time line. Every prophecy I traced happened exactly when the prophets had forewarned it would.
He taught me to look to the Greek before anyone ever put out an NIV or an RSV.
When I finally graduated and got out of his hair long enough to pursue college credits I'm sure he was relieved, though he was far too kind to have ever said so.
I continue to search for the truth.
When people told me "Nazareth doesn't exist," I spent a few weeks in libraries, and talking to professors. I was thrilled to find out that they found a "fishing village" with a plaque that said "Nazareth," right where Christ said it would be.
Things the nay-sayers hate to admit, is that no other historical document in the entire history of the world has as much supporting evidence as the bible has.
Look at the old testament - The Amarina Letters of 16th Century written by Canaite scribes show the social, political and religious relationships between Canaan and Egypt - and support the biblical recordings of the same.
Amenemope's Wisdom, an Egyptian text from the 1st millennium BC has 30 chapters that refelct the wisdom of Proverbs, and provding the closest external parallels to the Old Testament of any outside literature.
Ever study Gilgamesh in school? Is Akkadian, from the 2nd millennium B.C.. It is, for the most part, the adventures of Uruk, but it contains the Great Flood. Compare Gilgamesh to Genesis 6-9.
The 7 years of famine recorded in our bible is also recorded in Egyptian Records that date back to 2nd century B.C. These records state that Egypt experienced 7 years of low Niles and famines, and that a contractual agreement was made between Pharoah Djoser (28th Century B.C.) and "a god", to provide 7 years of prosperity. Compare this to Genesis 41.
Shishak's Geographical List - an Egyptian Document of the 10th Century B.C. is where Pharoah Shishak lists the cities he captured or made tributary during his campaign into Judah and Israel. Compare it to 1 Kings 14:25 and 26.
The Akkadian "Nabonidus Chronicle" of the mid 6th century b.c., tells about the absence of Nabonidus from Babylong, and how his son Balahazzar was the regent in charge. Compare that to Daniel 5:29-30.
There are, that I know of, at least 39 similar non-biblical documents that support the biblical accounts, or confirm the bible's description of social structures, or politics of the age. You would think that would convince an agnostic, but it doesn't. They just say, "That proves it is a legend." Which I will never understand.
No other document has as much "cross referenced" historical evidence as the bible.
Then, to look at the bible itself - It is written, literally over a span of centuries, and records thousands of years of history. There are 66 books if you look at the King James, and they are written by people from all walks of life - fishermen, Scholars, Kings, simple men who were prophets.. BUT - they all tell the same story. They don't contradict one another, they all tie in, God is unchanging in all that time.
Then there is, despite what the nay sayers tell you, archeological evidence.