The testimony of the the Holy Spirit is more excellent than reason or nature. General revelation leads us only so far but no further – that there is a God. God alone is a perfect witness of himself in his Word, and his Word only finds acceptance in our minds and hearts when it is sealed by the inward witness of the Spirit. The sheep hear the voice of the Shepherd and they follow him.
I like how Calvin put it: “those whom the Holy Spirit has inwardly taught truly rest upon Scripture, and that Scripture indeed is self-authenticated; hence , it is not right to subject it to proof and reasoning. And the certainty it deserves with us it attains by the testimony of the Spirit. For even if it wins reverence for itself by its own majesty, it seriously affects us only when it is sealed upon our hearts through the Spirit. Therefore, illumined by its power, we believe neither by our own nor by anyone else’s judgment that Scripture is from God; but above human judgment we affirm with utter certainty (just as if we were gazing upon the majesty of God himself) that it has flowed to us from the very mouth of God by the ministry of men. We seek no proofs, no marks of genuineness upon which our judgment may lean, but we subject our judgment and wit to it as to a thing far beyond any guesswork!”