Westminster Larger Catechism question number four:
Q. 4. How do we know that the Scriptures are the Word of God?
A. The Scriptures reveal themselves as the Word of God by their majesty and purity; by the harmony of all the parts together and with the scope of the whole, which is to give all glory to God; by their light and power to convince and convert sinners and to comfort and build up believers to salvation; but the Spirit of God bearing witness by and with the Scriptures in the heart of a person is the only thing able to fully persuade him that they are the very Word of God.