A student asked me this very good question the other day and so I thought I would put some thought to it.
The problem we have as Christians is that we work in a certain degree of circularity. We argue that the Bible points us to God and then determine who God is from the document. Can we therefore make the claim that the Bible is the Word of God? For me, it is a cumulative argument like so many theological arguments i.e. no one argument proves it, but the sum-total of a set of arguments points strongly in this direction. Having said that, it has to be conceded that this like so much of what theologians and philosophers postulate is not able to be proved totally in an emperical sense. Rather, it is a one faith-based conclusion to a set of thoughts.
1. The Bible's Self-Witness: There is no statement in the Bible that says that the whole Bible is the Word of God but there are a series of statements in which the Bible says things of itself that point in this direction.
For example, Paul writes in 2 Tim 3:16 that 'All Scripture is God-breathed' i.e. inspired by the Spirit of God. 'All Scripture' here includes the Old Testament writings as established at the time and accepted by the first Christians as authoritative. However, it may also refer to writings such as Mark's Gospel or proto-gospels and even some of the early letters of Paul.
Another example is Heb 4:12-13 where the unknown writer says that 'the Word of God is living and active, sharper than a double edged sword...' Similarly Paul describes the Word of God' as the sword of the Spirit (Eph 6:17). This points to the early Christians believing that the written word was inspired by God.
That it is legitimate to apply this notion 'word of God' to the NT writings is reinforce in Peter's comments on Paul's writings in 2 Pet 3:15-16 where he states: 'Bear in mind that our Lord’s patience means salvation, just as our dear brother Paul also wrote you with the wisdom that God gave him. He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction.' Note that Paul's letters are classified here as Scriptures indicating that early Christians quickly classed his letters as the Scriptures.
2. The Canonisation Process: The early Christians after the resurrection quickly saw the apostles as God-appointed spokespeople not unlike OT prophets. The books that were included in the NT were those that were apostolic (either by an apostle or linked directly), dated in the first generation after Christ, and which were theologically aligned to apostolic theology. There was dispute about some but the majority of the NT was set in place by the end of the first century. The Scriptures were seen from the beginning as the inspired Word of God in the same manner as the OT. We believe as Christians that the Spirit oversaw the process through human agency and the NT was set in place as the primary documents inspired by God through which God speaks his word today. Most importantly they are the Word of God because through the writings of the word of God we can find the living Word of God.
3. Its self-fulfilment: I am amazed at the fulfillment of the OT prophecies. The OT predicted that the saviour would come from the tribe of Judah, would be born in Jerusalem, would be born of a virgin (Greek LXX), would be known as the Son of Man, would do miracles, would ride into Jerusalem on a donkey, would suffer and die a dreadful painful death (Ps 22 is an amazing description of crucifixion), would have his clothes gambled for, would be pierced, would be buried with the rich, would rise from the dead and not see decay, would be the light to all nations and more. The Book of Daniel is set in the exile and points to the sequence of world kingdoms that follow it with uncanny accuracy. Although Christians debate this, for me the restoration of the nation of Israel and Jerusalem (1948, 1967) was an astonishing modern day fulfillment of the hopes of the OT prophets which were not completely fulfilled in the return from exile. In addition, the NT speaks of a rise in terrible suffering for the nations including famine, war, earthquakes, plagues and pestilences, the tossing of the sea etc at the climax of human history. These things are certainly occuring to me and as such we are nearing the climax of human history, the return of Christ.
4. Its coherancy: The Bible is a great coherent story from start to finish. It speaks of creation, fall, the call of a family and nation, that nations ups and downs, the hope of a Messiah-saviour, the coming of the saviour, his ministry and self-sacrifice to save the world, his resurrection, the coming of the Spirit to inspire those who believe to spread the message; God has come, salvation is here. And now we are in the age of the church, continuing the mission of God to see the world believe. I love its coherancy. Despite it being 66 books written over a period of 1500 years with many authors, it is one story that flows beautifully together with unbelievable coherency and consistency. It speaks as one work that God has governed with his Spirit.
5. It Explanation of Reality: One of the reasons I became a Christian is that I realised that the world-view undergirding the Bible is cogent reason to follow it. There are many angles on this. Here are some. Firstly, it explains our existence which to me demands a creator. Secondly, it explains the importance of humanity as God's image bearers and the apex of his creation. Thirdly, it explains the problem with the world i.e. sin and the inability of humanity to stop doing evil. Fourthly, it gives a coherent vision of salvation found through a means that allows humanity freedom, involves God in saving us through the cross and experiencing the Spirit when we believe. Fifthly, it gives all followers a vocation, to work for God and his purposes to promote good and hope through love. Sixthly, its ethic is brilliant; LOVE! Not only is love encouraged it is demonstrated in a God who through love, created and save. Seventhly, it gives hope that evil will be vanquished and we will experience eternal life through Jesus in untainted glory. Eighthly it is not some mythical nonsense but is rooted in human history that can be cross-referenced and checked out; based on the supreme event in human history, the resurrection. I could go on, but you get the point. There is no other explanation of reality that rings true to me and explains origins and gives such wonderful hope.
6. Its impact: The Bible is the world's best seller. Within 3oo years it had transformed the most powerful military nation in the world, the Roman Empire, not through military might but through love, martyrdom and proclamation of its central message; that Jesus is saviour and Lord of the world. Through it billions have come to know God through his agent Jesus Christ whose story lies at the centre of the Bible story. It then inspired generations of the Roman world. In recent centuries it has spread throughout the world as is the largest and most influential religion in the world. Its ethic has undergirded western civilisation.
So for me the Bible is the written Word of God through which the living Word of God is found and experienced. Not all experience him when they read it, but they are forced to decide one way or another. It is the vehicle through which God speaks. Its centre-piece is the four-fold Gospels which hold it together and have primacy in terms of understanding God and who he is because they tell the story of the coming of God's Son, Saviour and Lord. So read it and hear the Word of God.