The main foundation of all Christianity is the Bible, and it is direct revelation of God to man. Not  preachers, Popes, commentaries or church approved books, poetical prayers, magazines, websites, further revelations, or ANYTHING else can truly speak for Christianity except for the Bible. Now, there are ways of interpreting the Bible, but the Bible is and should be the ultimate and final authority in a Christian's life.

Now, what is the Bible? The Bible is 66 books written over thousands of years which were God's direct revelations to man. It was written by dozens men over millenniums, yet still agrees in every way. The Bible itself says it was written by God to man, in the way a person might hire a ghostwriter to write a book for them. 

2 Timothy 3:16 - All scripture [is] given by inspiration of God, and [is] profitable for doctrine, for reproof, for correction, for instruction in righteousness:

2 Peter 1:21 -For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.

Also, no one man or even an angel can glory in the Bible. No angel interfered or was a transfer for the Word of God, nor has any one man gotten sole glory, though men like Moses and the Apostle Paul are known for writing certain large amounts.

God also promised He would always preserve His word.

Isaiah 40:8 - The grass withereth, the flower fadeth: but the word of our God shall stand for ever.

Psalm 119:89 - For ever, O Lord, thy word is settled in heaven.

1 Peter 1:25 - But the word of the Lord endureth for ever. And this is the word which by the gospel is preached unto you.

How can we know the Bible is true? There are many ways we can know, which I would love to write another book about. It has been proven that there are Bibles up to 100 years after Jesus was alive, and nearly all of them are 100% to 95% accurate, all changes which are proven to be of no change in basic doctrines. Anyone who thinks the Bible has been changed, I would encourage to research the topic more. Knowing more is never a bad thing, and neither is keeping an open mind.

The Bible is also very accurate in areas of prophecy.

Israel would again be a nation. History shows Israel was seperated for hundreds of years and came back again the same people, exactly as the Bible prophesies.

Hosea 3:4-5 For the children of Israel shall abide many days without a king, and without a prince, and without a sacrifice, and without an image, and without an ephod, and without teraphim: Afterward shall the children of Israel return, and seek the LORD their God, and David their king; and shall fear the LORD and his goodness in the latter days.

Men would one day have knowledge be increased (internet) and move to and fro on the earth (cars and airplanes.) This has been slowly growing since Jesus Christ left earth.

Daniel 12:4 But thou, O Daniel, shut up the words, and seal the book, even to the time of the end: many shall run to and fro, and knowledge shall be increase.

Daniel also has a lot of prophesies that perhaps I will list at another time, and which I encourage you to study on your own!

The Bible is also always eventually proved true in areas of history. Many historians said that the Hittites were a myth, but the Hittites were later found to be true.

Genesis 15:20 And the Hittites, and the Perizzites, and the Rephaims,

Many have tried to destroy or have mocked the Bible, such as Voltaire. He said that soon the Bible would be regarded as lore, but after his death, his home was bought and used to print Bibles. Diocletian vowed to destroy all Bibles and even thought he had succeeded, but it is very clear that he didn't.

I do not believe these things were matters of coincidence, but rather the works of God, all which prove the credibility of the Bible as the unchanged Word of God.