1689: 1.5. Proof that the Scripture is from God
5. The testimony of the church of God may stir and persuade us to adopt a high and reverent respect for the Holy Scriptures. Moreover, the heavenliness of the contents, the power of the system of truth, the majesty of the style, the harmony of all the parts, the central focus on giving all glory to God, the full revelation of the only way of salvation, and many other incomparable qualities and complete perfections, all provide abundant evidence that the Scriptures are the Word of God. Even so, our full persuasion and assurance of the infallible truth and divine authority of the Scriptures comes from the internal work of the Holy Spirit bearing witness by and with the Word in our hearts.8
8John 16:13,14; 1 Corinthians 2:10–12; 1 John 2:20, 27.
Why do I believe and trust the Bible as God's written word to man? The answer to this question is way less complicated than you might think. Primarily I believe the Bible is God's Word because God saved me. St. Augustine once said, 'before I can understand, I must believe'. This is so true of the Christian life. This answer may not satisfy a skeptic, but the answer isn't less true because of skepticism. The framers of the Confession had the same answer. Primarily they believed in the authenticity of the Scripture because they were fully persuaded 'of the infallible truth and divine authority of the Scriptures'. Why? Because of the 'internal work of the Holy Spirit'. This was a slap in the face to Roman Catholicism who devalued the authority of Scripture by elevating the church; pope; etc. So primarily we know the Bible is God's written word because God gifted us with salvation.
Secondarily, we have the testimony of the church, the harmonization of all 66 books testifying to the exclusivity of God's gospel and the singular goal of giving God all praise and glory.
So to recap, the Holy Spirit testifies to the Scripture being God's Word, the Bible itself claims divine authorship (see previous post), the cohesive narrative of all 66 books supports this, and Christians all throughout church history have affirmed this.
So believer, be confident in our Holy Scriptures. God is the ultimate author and He has persevered it for thousands of years and will continue to preserve it as He has promised.