That Scripture is the word of God, says Calvin, was not established by the church but was certain prior to the church’s decision, for the church is built on the foundation of apostles and prophets. Scripture brings with it its own authority; it is self-based and self-attested as trustworthy (αύτοπιστος). Just as light is distinguished from darkness, white from black, sweet from bitter, so Scripture is recognized by its own truth. But Scripture acquires certainty as God’s own Word with us by the testimony of the Holy Spirit. Though proofs and reasonings are of great value, this testimony surpasses them by far; it is more excellent than all reason. Just as God can only witness concerning himself in his Word, so his Word does not find belief in the hearts of human beings before it is sealed by the internal testimony of the Holy Spirit. The same Spirit who spoke through the mouths of the prophets must work in our hearts to persuade us that they faithfully proclaimed what had been commanded by God. The Holy Spirit, accordingly, is the “seal” and “guarantee” for confirming the faith of the godly. If we have that testimony within us, we do not rest in any human judgment but observe without any doubt as if we were gazing upon God himself in it–that Scripture came from the mouth of God through the ministry of human beings. We subject our judgment to it “as to a thing far beyond any guesswork!”
Reformed Dogmatics Vol. 1: Prolegomena (Grand Rapids, Michigan; Baker Academic; 2003) p. 583.