Why is the Bible a foundation of theological evidence? Please excuse my ignorance I'm new to this.

I've been looking through the Q&amp;A on the site and I've noticed scripture and especially passages from the Bible are cited as evidence for one argument or another.

What is the theological argument that the Bible should form the basis of the Christian belief system?

EDIT

Whilst Mason's answer gives a good description of the ways, in which, the Bible is a good guide to commonly held Christian beliefs, the answer is, as Mason hints, a tautology.

I'm still a bit confused. There must have been a time after Jesus when some parts of the Bible had not been written and the Bible had not been gathered into one. Why is the Bible or, why has the Bible become the central text of the Christian faith?
Wow, this is a really fundamental question.  And the basic answer is, because the Bible is the foundation of Christian theology.

I hope that doesn't sound like a tautology.  What I mean by that is that the Bible is where we find the description of what Christianity is and what it's about.  The first few chapters of Genesis give us the creation of the world, the introduction to God and mankind, and the fundamental problems of our existence: sin and death.  The rest of the Old Testament teaches us about God's chosen people, his relationship with them and the covenants he made with them, and their history.  It places particular emphasis on how things worked out for them when they did and when they didn't live according to God's commandments.

And then we come to the New Testament, where the solution to the fundamental problems introduced in Genesis is given.  God sent his son, Jesus, to earth to pay the price of sin and overcome death, to make it possible for mankind to be restored to a perfect state through faith in him and his atonement.  It explains how Jesus did that, and what he taught us about how to live in order to make the atonement apply to us.  Then it continues on, showing the founding and early growth of the church that Jesus organized, based on the writings and the travels of the apostles that he ordained as leaders of the church.  They went around teaching people, organizing and building up the church, and clarifying various points of doctrine for people who didn't understand all the details.

This content makes the Bible a solid foundation for understanding the basis of Christianity and Christian doctrine, and the first authoritative place to look when searching for theological evidence on questions related to Christianity.