For those of us who believe that the Bible is truly the Word of God, it is difficult to explain to the skeptic why we believe something just because the Bible says that it is true. The skeptic cannot conceive of how someone can have that kind of faith in a book written by fallible human beings.
To quote the apostle Paul, in 1 Corinthians 1:18-25 (NKJV):
"18 For the message of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God.
19 For it is written:
“ I will destroy the wisdom of the wise,
And bring to nothing the understanding of the prudent.” [cf Isaiah 29:14]
20 Where is the wise? Where is the scribe? Where is the disputer of this age? Has not God made foolish the wisdom of this world?
21 For since, in the wisdom of God, the world through wisdom did not know God, it pleased God through the foolishness of the message preached to save those who believe.
22 For Jews request a sign, and Greeks seek after wisdom;
23 but we preach Christ crucified, to the Jews a stumbling block and to the Greeks foolishness, 24 but to those who are called, both Jews and Greeks, Christ the power of God and the wisdom of God.
25 Because the foolishness of God is wiser than men, and the weakness of God is stronger than men."
For believers in the divinely inspired, inerrant Bible, the following passage merely corroborates what we already knew:
"All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness." 2 Timothy 3:16 (NKJV)
A source of disagreement among Christians of certain denominations, is the concept of societal "norms" dictating how the Bible should be interpreted. Considering what God says about Himself, there is no reason to think that what He says are right and wrong should change, just because customs change.
"God is not a man, so he does not lie. He is not human, so he does not change his mind. Has he ever spoken and failed to act? Has he ever promised and not carried it through?"Numbers 23:19 (NIV)
“ 'For I am the LORD, I do not change.' "Malachi 3:6a (NKJV)My personal belief is that the Bible is the only completely trustworthy authority on how to live a life pleasing to God. Since the primary focus on my life is to develop a relationship to God through Jesus Christ, then the Bible is my manual for living.