Q. How does it appear that the Scriptures are the Word of God?
A. The Scriptures manifest themselves to be the Word of God, by their majesty (Hos. 8:12; 1 Cor. 2:6–7, 13; Ps. 119:18, 129) and purity (Ps. 12:6; 119:140); by the consent of all the parts (Acts 10:43; 26:22); and the scope of the whole, which is to give all glory to God (Rom. 3:19, 27); by their light and power to convince and convert sinners, to comfort and build up believers unto salvation (Acts 18:28; Heb. 4:12; Jam. 1:18; Ps. 19:7–9; Rom. 15:4; Acts 20:32): but the Spirit of God bearing witness by and with the Scriptures in the heart of man, is alone able fully to persuade it that they are the very word of God (John 16:13–14; 1 John 2:20, 27; John 20:31).
Chapter I: Of Holy Scripture
IV. The authority of the Holy Scripture, for which it ought to be believed, and obeyed, depends 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.
For more on the subject, please visit the Systematic Theology page.