First, we must recognize that the biblical books have internal qualities that demonstrate that they are from God. Just as natural revelation (the created world) has characteristics that show God is the author of nature (Ps. 19; Rom. 1:20), so we should expect special revelation (Scripture) to have such characteristics that show God is its author.
One example is the efficacy and power of Scripture. It’s not just that Scripture says things, but the Scripture does things. It convicts (Heb. 4:12–13), it encourages (Ps. 119:105), it comforts (v. 50), and it brings wisdom (v. 98). In short, this book is alive. Even more than this, the Bible brings understanding in regard to the biggest questions of life (v. 144). It provides a coherent and compelling worldview that explains reality like no other book.
Another example is the unity and harmony of Scripture. It is incredible to behold how so many different authors—writing in different times, locations, and cultures—can weave together a single, unified, coherent story of the redemption of all things through Christ. Such harmony is not man-made but evidence of a divine origin.
In short, Christians know the Scriptures are God’s Word because in them they hear the voice of their Lord. As Jesus declared, “My sheep hear my voice, and I know them, and they follow me” (John 10:27).
In addition to the internal quality of these books, we can also look to the historical origins of these books as evidence of their unique character. These books come from God’s authenticated messengers, prophets and Apostles who were authorized to speak for Him. The Scriptures contain not just the words of human beings but the words of human beings who were called to be God’s mouthpieces (2 Peter 1:21).
Of course, we are not always certain of the particular author of every biblical book (for example, the book of Hebrews). But, even in such cases, we have solid historical evidence that situates these books in time periods and circumstances where we know God was actively working among His people to reveal His Word.
Moreover, it should be noted that the books of the Bible have been subjected to the keenest scrutiny and the most rigorous examination by modern scholars. And time and time again, the books of the Bible have proved themselves to be historically reliable and worthy of our trust.