Basic Christian doctrine will tell you that the Bible is the word of God, but there's a lot of confusion over exactly what this means. A child might imagine God sat down and wrote a book, and some may satirise it as if that's what we believe. However, its main authors are human.
That's not to say there aren't any parts that weren't written by God Himself. The Decalogue, for example, was written on two stone tablets by God's own finger (Exodus 31:18), and thus, when we read them, we are reading words literally written (originally) by God. But the majority of Scripture was written by human hands. Sadly, this has caused a major surge in irreverence over the past few centuries. We have seen the rise of red letter Bibles, and with it, red letter Christians. "That's just Paul", they say when confronted with Biblical truths they cannot handle. "Jesus never said that."
But even the red letters affirm the black. Jesus strongly confirmed the Scriptures, both the Old Testament to which He appealed, and the New Testament which He certified. The Old Testament, He constantly said things like "Have you not read..." (e.g. Matthew 19:4), "it is written..." (e.g. Mark 14:27), "Are you not therefore mistaken, because you do not know the Scriptures..." (Mark 12:24), "...the Scripture cannot be broken..." (John 10:35) etc. He even said "For if you believed Moses, you would believe Me; for he wrote about Me. But if you do not believe his writings, how will you believe My words?”" (John 5:46-47).
The New Testament, He certified by affirming the messengers. He told His Disciples "He who receives you receives Me, and he who receives Me receives Him who sent Me." (Matthew 10:40), and promised them the Holy Spirit would "...teach you all things, and bring to your remembrance all things that I said to you." (John 14:26), and "...guide you into all truth..." (John 16:13). Thus, if you have any respect for Jesus, you must have that same respect for the rest of Scripture. This is hardly surprising, given that Scripture happens to be the only reliable source on Jesus, as well as the only primary source. If you don't trust the Bible, how can you trust what the Bible says about Jesus? And if you're not getting your information about Jesus from His word, where are you getting it from? Thus, the only way to consistently reject the Bible is to reject Jesus.
But how can the Bible be the word of God if it wasn't actually written by Him? Two particular Scriptures help us out here, the first being 2 Peter 1:19-21: "And so we have the prophetic word confirmed, which you do well to heed as a light that shines in a dark place, until the day dawns and the morning star rises in your hearts; knowing this first, that no prophecy of Scripture is of any private interpretation, for prophecy never came by the will of man, but holy men of God spoke as they were moved by the Holy Spirit."
This comes after Peter has affirmed his own reliability as a direct and fellow witness of his message. He wasn't following what he called "cunningly devised fables". Rather, he and his fellow Apostles saw, with their own 22 eyes, the things described in the Scriptures. They saw Jesus' miraculous ministry. They saw Him arrested, betrayed by one of their own, and hung on a cross. They saw Him die. And they saw Him alive again. Thus, Peter says "we have the prophetic word confirmed". He then goes on to describe the source of this prophetic word.
See, human beings can make predictions about the future. That's why we occasionally trust the weather forecast. That's why we can say, with reasonable accuracy, who is going to win an election. That's why we mark dates in our calendars, even if it's something as unpredictable as a due date for a baby. We make predictions, and they are often successful. But because we lack God's foresight, we also make dog's dinners of the practice. We get wet, because the weatherman told us not to bother with an umbrella. We still go out and vote, because even if we're confident our guy will win, we want to make sure of it. And frankly, babies aren't bound to our time scales, and they have a habit of letting us know it.
But prophecies are not of any private interpretation. In fact, the prophets themselves very often did not know the meanings of their own prophecies (1 Peter 1:10-12). They didn't make observations about their world and assume "hmm, I think God's gonna make a virgin conceive, and that child is gonna die on a cross and rise again". No, everything they wrote, they did so as the Holy Spirit moved them.
In Greek, that word translated "moved" is pherō (φέρω), which is the same word found in verses like Acts 27:17, which says "When they had taken it on board, they used cables to undergird the ship; and fearing lest they should run aground on the Syrtis Sands, they struck sail and so were driven." Thus, it seems, inspiration is a lot like the wind guiding a wooden ship. It didn't go where the human authors wanted it to go, it went exactly where God "blew" it.
This seems appropriate, given the other main passage on inspiration. In 2 Timothy 3:14-17, we read "But you must continue in the things which you have learned and been assured of, knowing from whom you have learned them, and that from childhood you have known the Holy Scriptures, which are able to make you wise for salvation through faith which is in Christ Jesus. All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work."
The word in verse 16, translated as "given by inspiration of God", is theopneustos (θεόπνευστος), which more literally translates to "God breathed". Thus, when you read the Scriptures, you are effectively reading what God Himself has spoken.
But it's clearly not as if God sat in Heaven dictating the words of Scripture. Each Biblical author shines through in their own works. Their personalities, their desires, at one point Paul even says "...to the rest I, not the Lord, say..." (1 Corinthians 7:12). So is this part of Scripture not actually from the Lord?
The answer to this dilemma is found in an acceptable distinction between what God says out loud and what God delivers through less explicit means. In context, Paul draws on a rather explicit teaching delivered by Christ on the subject of marriage to make his point in 1 Corinthians 7:10-11. This teaching of Christ is recorded in Matthew 19:1-12 and Mark 10:1-12.
But what he goes on to say in 1 Corinthians 7:12 can be found nowhere else in Scripture. What can be found in Scripture, however, is the concept of "general" revelation. Psalm 19:1, for example, tells us the Heavens declare the glory of God, and the skies declare His handiwork. This concept is later repeated in Romans 1:18-21, where Paul describes the folly of men who reject what God has shown them, not by visions or by Scriptures, but by creation.
But doesn't that go against what Peter says earlier about no prophecy of Scripture being of any private interpretation? This isn't actually what I'm saying here. I'm not saying Paul wrote Scripture based on his observations of the natural world. My point here is only to say God can speak without speaking, and thus it is legitimate to distinguish between what the Lord explicitly says with His own expressed words and that which He says through other means.
With this in mind, consider what Peter says about him in 2 Peter 3:14-16: "Therefore, beloved, looking forward to these things, be diligent to be found by Him in peace, without spot and blameless; and consider that the longsuffering of our Lord is salvation—as also our beloved brother Paul, according to the wisdom given to him, has written to you, as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught and unstable people twist to their own destruction, as they do also the rest of the Scriptures."
Note how Peter identifies Paul's epistles as Scripture, which should be heeded, and if twisted or ignored, bring destruction. But Peter doesn't say God attached strings to Paul's arms and made him into a puppet. He says that Paul wrote these things "according to the wisdom given to him".
Which brings us full circle to the purpose of the Holy Spirit. Paul, in no way inferior to the other Apostles (2 Corinthians 11:5), had the same access to the Holy Spirit. This means He had an infallible guide, leading him into all truth. Thus, he wrote infallible Scriptures in spite of his own fallibility. His personality was preserved, his flaws were not.
Thus, inspiration is a fantastic cooperation of man and God to produce the perfect document of communication from God to man. The Bible is the word of God in the sense that He was in full control of what went into it, as surely as if He, Himself, held the quill. But the quills He used, rather than being feathers on a stick, were instead living, breathing, human beings, telling us what He wanted us to know. These human beings only served to glorify God, rather than hinder His message.