How do we know that what is written in the Bible are God’s word? This is a question of authority.
As I began to explain in “The Bible, Introduction / Canon,” all the words in the Bible are God’s words.
Throughout the Scriptures, the text claims that it is indeed the word of God . In the Old Testament (OT) you will read, “Thus says the Lord…,” over and over again. When the prophets used this phrase they were claiming to be bringing an authoritative word from God. Additionally, we are told that God speaks “through” his prophets (Jeremiah 37:2 and many others). If a “prophet” used the phrase “thus says the Lord,” and what they said was not from God (the false prophet would have been found out because their words would not have come to pass) they would face corporal punishment which ended in death (Deuteronomy 18:15-22).
Paul, in 1 Timothy 3:16, affirms the OT was “God breathed” in its origin. Second Peter 1:21 reminds readers that no prophecies (the OT) were written down by the “impulse of man,” but that they came from the impulse of God as men were moved by the Holy Spirit. Thus, all the scripture in the OT has been attributed to God.
The New Testament (NT) also affirms itself as the words of God. In 2 Peter 3:16, Peter indicates that the letters of Paul are included in the “Scriptures.” Thus, Peter and the early church would have considered the writings of Paul on the same level as all the other Scriptures they possessed. Additionally, Jesus promised the disciples that the Holy Spirit would help them remember all the things that He had said to them, seemingly for the purpose of recording the sayings (John 14:26).
This brings up a good question, “How did God give His message to His people?” Hebrews 1:1 tells us that there are in fact “many ways” God has given His words to His people. At times, God audibly spoke to the author and they wrote what they heard (Revelation 2:1). Other authors took time to interview and research what God had done or said then wrote about the reports (Luke 1:1-3). And, as already mentioned, the Holy Spirit would bring to mind the things that needed to be recorded (John 14:26). Through the writing, the reader can identify the personality and character of the authors. However, God chose them and guided them so that the words they used were His words.
If the Bible is God’s word, and God is the highest authority, then the scripture can be trusted. As such, they are truth and the gauge by which all truth is to be measured. Some have argued that the Bible has things that seem to contradict itself or other know facts. The key word being “seem.” While there are times when it seems to contradict in this way, the reality is that no one has successfully proven that a REAL contradiction exists.
As you read the Bible you become more and more convinced that it is in fact the word of God. Paul described this process in 1 Corinthians 2:14-15. Without the help of the Spirit, one may conclude that the Bible is just another book; but, to those whom are seeking the guidance of the Spirit, they recognize the truth of the claims of the scripture. Interestingly, when I have conversations with people about theology they never question the authority of the scripture to answer the questions about God.
Wayne Grudem says this about the Bible:
…that the Bible is historically accurate, that it is internally consistent, that it contains prophecies that have been fulfilled hundreds of years later, that it has influenced the course of human history more than any other book, that is has continued changing the lives of millions of individuals throughout its history, that through it people come to find salvation, that it has a majestic beauty and a profound depth of teaching unmatched by any other book, and that it claims hundreds of times over to be God’s very words.
If we believe that the Bible is the Word of God, then it has authority over our lives. This means that any disobedience to the proclamations in it that are directed toward believer are direct disobediences to God.
What do you think about the authority of Scripture?
 We must realize that God did not audibly speak every word, but did inspire the words that were used.