How is the Bible evidence for God's existence?

When people point out contradictions of the Bible, they are told it has been tainted by man. When people argue against the idea that the universe was created 6000 years ago, they are told that part of the bible was tainted by man. Yet, christians believe the Bible to be the holy book! How can someone say that only parts of the Bible are correct? And even worse, how can you choose which parts are correct? This God is supposed to be unknowable, someone who no-one understands, how can you decide which parts of the Bible are right?