Westminster Confession, Chapter 1, Paragraph 4
4 – The authority of the Holy Scripture, for which it ought to be believed, and obeyed, dependeth not upon the testimony of any man, or church; but wholly upon God (who is truth itself) the author thereof: and therefore it is to be received, because it is the Word of God.
Paragraph 2 of the confession speaks about the books in the bible. And paragraph 3 speaks about the Apocrypha and other human writings that are not to be considered the words of God.
Paragraph 4 here speaks about authority. In our justice systems, it is common to have witnesses come to the stand and testify in court because they have witnessed some event. The more witnesses you have that are consistent to a fact, the more likely it is that their perspective is true. But what happens when something like the Bible is in the dock of the courtroom? There are no alive persons today that could state they were there when God wrote it.
Truth itself comes to the stand and witnesses that He was there and that He wrote it. Does the court need any more evidence? We do not need any fallible personal witnesses to this fact if truth himself states it.
2 Timothy 3:16a says “all scripture is given by inspiration of God …”
1 Thessalonians 2:13 “For this cause also thank we God without ceasing, because when ye received the word of God which ye heard of us, ye received it not as the words of men, but as it is in truth, the word of God, which effectually worketh also in you that believe.”