The origins of the Bible are often misunderstood, even among Christians. Of course, atheists claim that the Bible was written by man, and therefore cannot be trusted, but what's scary is that many Christians are just as quick to wave the Bible off in the same manner.
The first thing to note is that the Bible being written by man does not make it unreliable. In fact, the great irony is that whenever we read "the Bible was written by man", we are reading something that was also written by man. Thus, we end up with something similar to the Epimenides paradox. Epimenides, a Crete, said that Cretes always lie. But if Cretes always lie, it must be the case that Epimenides was lying when he said Cretes always lie. In the same way, if man writes "the writings of man are unreliable", then it must be unreliable that what man writes is unreliable.
Of course, we don't need this to be stated as a logical paradox to know that it is an unreliable statement. We rely on the writings of man all the time. Everything from a simple instruction booklet for how to assemble a new piece of furniture all the way to the books you used in school to teach you 90% of everything you will ever know, was written by men, upon whom we rely to perform the very purpose for which they wrote.
But obviously, men do make mistakes, so the argument still holds some water. Right? Wrong. See, although "the Bible was written by men" is a true statement, it is more technically accurate to say "the Bible was written by God through men". You may think of this as the difference between claiming the pen wrote the book and saying the author wrote the book.
The Bible tells us quite clearly that the ultimate author of scripture is not man, but God. Peter, for example, points to prophecy, which he says did not come about from one's own interpretation, but actually from the Holy Spirit, who spoke through men.
Modern men do have some prophetic capabilities. A meteorologist, for example, can examine the data and predict, to some degree of accuracy, what the weather will be like on a given day. A weather man is speaking from a human interpretation, which may be right, but of course, everyone knows it's best to take a raincoat, just in case.
This goes further than just the weather. Some people literally have the job of predicting the future in order to maximise profits, prevent terror attacks, or even just weigh in on who will win an election. These are all examples of men using their own interpretation of current events to predict future events. However, as everybody knows, these predictions don't always come to pass.
But whereas man's prophecies are all reasonably foreseeable events that might not even come to pass, Biblical prophecies are unforeseeable events that, by all rights, should never have come to pass. The Bible's prophecies are so incredible that those who doubt the possibility of prophecy actually attempt to post-date the books in which they occur. This is something that could only happen if God Himself foretold the events.
Further evidence for divine inspiration is the unity of scripture. Imagine you have a room of 10 people, from the same nation, during the same era, even working in the same profession. Assuming you asked them all for their opinions on one controversial issue, what do you imagine your chances are of getting the same reply? I would imagine some of my readers have already debated their own coworkers on some issue or another. The chances of finding just 10 people, regardless of how similar their backgrounds, who agree on everything, are slimmer than a starving stick insect.
And yet, the Bible has 4 times as many authors, living in different regions, during different eras, even speaking different languages, and yet the Bible is such a unified book that it actually ends up uniting people who would otherwise be divided. When 40+ authors speak as one, it's fair to accept their claim to have been moved by one immortal spirit.
Which brings us to our final point. The Bible was, indeed, written by men of God. We're not just talking about people who studied under a guy who studied under a guy who studied under a guy. We're talking about men who knew God. They saw Him in some form, they heard Him speak in some form, the Apostles even had the privilege of eating, drinking and sleeping with the Lord Jesus Christ. If any human beings had the right to give their opinions on God, it would be them. Even their uninspired teachings would hold some weight. My brethren, if you believe these men did not have an adequate understanding of God, then neither do you. Under the correct Christian view of divine inspiration, people can come to know God again and again for generations to come. But under the belief that the Bible is just man's "best guess", you have no authoritative source of doctrine, or theology. You are inevitably forced to believe that you have less hope of knowing God than an iron age fisherman. Is that really the position you want to find yourself in? If you have to choose between trusting the word of God, or never knowing Him, it shouldn't take some dude on the internet to tell you to pick the first option.