Evangelical Christianity is grounded in the Protestant view of Scriptures. In particular that the Scriptures are uniquely the Word of God, having been inspired of the Holy Spirit in the original autographs. Christian doctrine and practice are grounded in this fundamental presupposition. Included in inspiration is the idea that God has adequately, accurately and understandably revealed himself and necessary truths about himself, his character and about reality.
It is all well and good for Christians to operate with this assumption, but is this assumption warranted? In this article, Is the Bible Authoritative, Greg Koukl gives a great summary argument that the believer has reliable warrant for it, as well as a good argument against the complaints of those who attack it based on the fact that it has fallible human authors.