Concise Theology by J.I. Packer
But you have an anointing from the Holy One, and all of you know the truth. 1 JOHN 2:20
Why do Christians believe that the Bible is the Word of God, sixty-six books forming a single book of instruction in which God reveals to us the reality of redemption through Jesus Christ the Savior? The answer is that God himself has confirmed this through what is called the inward witness of the Holy Spirit. In the words of the Westminster Confession (1647):
We may be moved and induced by the testimony of the Church to an high and reverend esteem of the Holy Scripture. And the heavenliness of the matter, the efficacy of the doctrine, the majesty of the style, the consent of all the parts, the scope of the whole (which is, to give all glory to God), the full discovery it makes of the only way of man’s salvation, the many other incomparable excellencies, and the entire perfection thereof, are arguments whereby it doth abundantly evidence itself to be the Word of God: yet notwithstanding, our full persuasion and assurance of the infallible truth and divine authority thereof, is from the inward work of the Holy Spirit bearing witness by and with the Word in our heart. (I.5)
The Spirit’s witness to Scripture is like his witness to Jesus, which we find spoken of in John 15:26 and 1 John 5:7 (cf. 1 John 2:20, 27). It is a matter not of imparting new information but of enlightening previously darkened minds to discern divinity through sensing its unique impact—the impact in the one case of the Jesus of the gospel, and in the other case of the words of Holy Scripture. The Spirit shines in our hearts to give us the light of the knowledge of the glory of God not only in the face of Jesus Christ (2 Cor. 4:6) but also in the teaching of Holy Scripture. The result of this witness is a state of mind in which both the Savior and the Scriptures have evidenced themselves to us as divine—Jesus, a divine person; Scripture, a divine product—in a way as direct, immediate, and arresting as that in which tastes and colors evidence themselves by forcing themselves on our senses. In consequence, we no longer find it possible to doubt the divinity of either Christ or the Bible.
Thus God authenticates Holy Scripture to us as his Word—not by some mystical experience or secret information privately whispered into some inner ear, not by human argument alone (strong as this may be), nor by the church’s testimony alone (impressive as this is when one looks back over two thousand years). God does it, rather, by means of the searching light and transforming power whereby Scripture evidences itself to be divine. The impact of this light and power is itself the Spirit’s witness “by and with the Word in our heart.” Argument, testimony from others, and our own particular experiences may prepare us to receive this witness, but the imparting of it, like the imparting of faith in Christ’s divine Saviorhood, is the prerogative of the sovereign Holy Spirit alone.
The illumination of the Spirit witnessing to the divinity of the Bible is universal Christian experience, and has been so from the beginning, though many Christians have not known how to verbalize it or to handle the Bible in a manner consistent with it.