Your Question: Why would I believe the Bible? How can I believe that what the Bible says is true?
If the Bible is the Word of God, then the Bible will be 100% true, infallible, perfect and without error (Psalm 19:7), because every word in the Bible comes from God (2 Timothy 3:16), and God cannot lie (Titus 1:2; Hebrews 6:18). Everything God says is good and true (Psalm 19:10).
So, the real question is: how do we know that the Bible is the Word of God? For Christians, the answer is simple: we believe the Bible is the Word of God, because Jesus Christ our Lord tells us that the Bible is the Word of God!
Attitude of Jesus to the Old Testament
First, consider Jesus’ attitude to the Old Testament. Jesus Himself quoted and obeyed the Old Testament (e.g. Matthew 4:4, 7, 10; 26:53-54), because Jesus always obeys God His Father (John 6:38). As Jesus says, God’s word cannot ever be changed, even by a single letter (Matthew 5:18). The Old Testament we have today is the same as the Old Testament that Jesus had 2000 years ago (Luke 24:44). The Old Testament, then, is the Word of God (Mark 7:5-13).
Jesus about the New Testament
Now, let’s consider what Jesus says about the New Testament. Jesus chose His apostles to be with Him for three years to SPEAK with Jesus’ authority (Mark 3:14); and at the end of Jesus’ earthly ministry, Jesus promised to send the Holy Spirit to help these eyewitnesses remember and understand everything Jesus had done and said (John 14:26; 15:26-27; 16:13-15), so that they would be able to WRITE with His authority. So, Matthew’s Gospel and John’s Gospel are the Word of God, because they are written by the apostles Matthew and John. The letters of Peter and John, and the book of Revelation, also were written by the apostles, and so they are also the Word of God. Peter is behind Mark’s gospel, so Mark’s gospel is the Word of God, too. Jesus also chose Paul to be an apostle (Acts 22:14-15), and the apostle Peter confirms that Paul’s letters are also Holy Scripture, so Paul’s epistles are also the Word of God (2 Peter 3:15-16). Paul, in turn, confirms that Luke’s writings are Holy Scripture, too (1 Timothy 5:18 – look at the footnotes), which means Luke and Acts are also the inspired word of God!
That’s why Christians believe that the Bible is the Word of God. Jesus, the Son of God, is our Lord and Savior, so we trust everything Jesus says; and Jesus teaches us that the whole Bible is the Word of God. So, if Jesus is right, then the whole Bible is also true.
No other explanation
However, even if you’re not a Christian, you can see just by reading the Bible that the Bible must be the Word of God: there is no other explanation! Consider, for example, the predictions made by the Old Testament prophets. They said that one day, a man would be born who would crush Satan (Genesis 3:15). We would recognize him, because he would be a Jew, from the tribe of Judah (Genesis 49:10), and a descendant of King David (2 Samuel 7:12-13). He’d be born to a virgin (Isaiah 7:15), in the village of Bethlehem (Micah 5:2), during the Roman Empire (Daniel 2:40-45). He would die for our sins (Isaiah 53), rise to life again (Psalm 16:9-11), be given a kingdom that would last forever (Daniel 7:13-14), and be worshiped by people in every nation of the world (Psalm 72:17). Who fulfilled all these prophecies? Jesus!
God is in charge of history
This makes us wonder: how did the Old Testament prophets know all these details about Jesus hundreds of years before He was born? They even described the details of his crucifixion before the Romans had even invented crucifixion (Psalm 22:12-18)! How did they know? Could it all have just been a lucky guess? No, there’s only one explanation: God is in charge of history, and God told the Old Testament prophets in advance what He was going to do (2 Peter 1:21). So when Jesus came, and everything happened exactly as the prophets had predicted, it proved that all that the Old Testament says is true (2 Corinthians 1:20). But if the Old Testament is true, and if the Old Testament teaches us to believe in Jesus (Luke 24: 25-27, 44-47; John 5:46-47), then Jesus must really be the Promised One (Acts 17:2-3), so we must believe and obey everything Jesus says and teaches (Acts 3:22-23).
In other words, Jesus and the Old Testament prove each other. Each proves that what the other one says is 100% true. And therefore, the Bible is the Word of God.