Biblical Christians follow what the Bible says because it is God's word: all of its teachings were given by the inspiration of God, so that all of it is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be perfect, thoroughly equipped to perform all good works (2 Timothy 3:16-17). It is only the Bible which is able to make humans wise unto salvation through faith which is in Jesus Christ (2 Timothy 3:15, Romans 10:17, Acts 13:48, James 1:18, 1 Peter 1:23-25).
One of the prime aims of the devil is to get humans to reject God's word the Bible and start believing something else which sounds better to humans (Genesis 3, Matthew 16:21-23), usually something which allows humans to continue in their lusts (2 Timothy 4:2-4, 1 Timothy 4:1-2) and which appeals to the devilish pride of humans, wanting to be as important as God (Isaiah 14:12-15, 1 Timothy 3:6).
Suppose for a second that a person came up to you and he says that he wrote a book that was inspired by God. Would you count this new book as a source on God?
If a person came up to Biblical Christians today and said that he wrote a book that was inspired by God, they would have to check its teachings against what God's word the Bible already says (Acts 17:11) and make sure that it didn't contradict anything in the Bible (2 Timothy 4:2-4). Also, even if if didn't contradict anything in the Bible, its teachings would still have to pass the test of Biblical Christians being able to (as it were) recognize the voice of Jesus Christ in what it was saying (John 10:27). Biblical Christians are able to recognize if something which is said is from Jesus Christ (John 10:4) or from "a stranger", and they will not follow the latter (John 10:5). Biblical Christians are able to do this because they have received God's own Holy Spirit (1 Corinthians 2:11-13), Jesus Christ's own mind (1 Corinthians 2:16). Non-Christians reject the Bible because they do not have God's Spirit, they do not have Jesus Christ's mind, but only their own, natural minds, which can only see the things of God as foolishness (1 Corinthians 2:14, 1 Corinthians 1:18), because of the wholly-corrupt nature of all humans in their natural state (Romans 3:10-12).
In anycase you follow the Bible because you believe the circumstantial evidence is enough?
Biblical Christians follow the Bible not only because of the direct, spiritual evidence referred to above, but also because of the direct written evidence in the New Testament, where Jesus Christ himself confirms that all of the Old Testament is true (Matthew 5:17-18, Luke 24:44-48). And all of the New Testament was written by eyewitnesses of Jesus Christ (2 Peter 1:16, 1 John 1:1-4, 1 Corinthians 9:1, John 19:35, John 21:24, Luke 24:48, Revelation 1:17-19) or their immediate followers (Luke 1:1-2, Hebrews 2:3). Also, no teaching in the Bible has ever been proven false. So there is no reason for any Christian to reject anything in the Bible.
Jesus says to Christians: "If ye continue in my word, then are ye my disciples indeed" (John 8:31). Christians must be willing to die before they would deny any part of his word: "For whosoever will save his life shall lose it; but whosoever shall lose his life for my sake and the gospel's, the same shall save it. For what shall it profit a man, if he shall gain the whole world, and lose his own soul? Or what shall a man give in exchange for his soul? Whosoever therefore shall be ashamed of me and of my words in this adulterous and sinful generation; of him also shall the Son of man be ashamed, when he cometh in the glory of his Father with the holy angels" (Mark 8:35-38).