There is a bluegrass gospel song “If you don’t believe the Bible, then you don’t know the Lord.”
I suppose it is possible (but highly unlikely) for a person to have a bias against the Bible due to their early training, then hear the gospel and become a Christian. For awhile they might think they don’t believe in the Bible, but they do and just don’t realize it, for all the gospel of Jesus Christ is found there, as are the scriptures that Jesus Himself regarded as authoritative. If such a person were to read the Bible and get proper instruction and discipleship, they would easily come to believe it However, some call themselves Christians but are not, and these people disbelieve, diminish, and seek to revise the Bible.
While the Bible is not our only authority, it is our <i>first</i> authority until the Lord returns in glory, because it contains the words of Jesus Christ and the scriptures that He valued as authoritative (“it is written…”). While of course Jesus interpreted them correctly for the people, He always upheld their authority as He does clearly in Matthew 5:17-19.
As Dr. John R.W. Stott put it: "We take our stand on the divine origin of the Bible, because we believe the Bible itself requires us to do so. Indeed, it is a strange fact that theologians who are prepared to accept the biblical doctrine of God, of Christ, of the Holy Spirit, of man and of the church, are often not willing to accept the biblical doctrine of Scripture. But if the Bible is authoritative and accurate when speaking about other matters, there is no reason why it should not be equally so when speaking about itself.”
And what does Scripture say about itself? A great deal. Here are only two NT witnesses (among many):
Peter: “And we have the word of the prophets made more certain, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts. Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation. For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit.” (2 Peter 1:20-21, NIV)
Paul: “...the Holy Scriptures, which are able to make you wise for salvation through faith in Christ Jesus. All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, so that the man of God may be thoroughly equipped for every good work.” (2 Timothy 3:16, NIV)