We are persuaded of the authenticity of Scripture by God alone

"Let it be considered, then, as an undeniable truth, that they who have been inwardly taught by the Spirit, feel an entire acquiescence in the Scripture, and that it is self-authenticated, carrying with it its own evidence, and ought not to be made the subject of demonstration and arguments from reason, but it obtains the credit which it deserves with us by the testimony of the Spirit. For though it conciliate our reverence by its internal majesty, it never seriously affects us till it is confirmed by the Spirit in our hearts. Therefore, being illuminated by him, we now believe the divine original of the Scripture, not from our own judgment or that of others, but we esteem the certainty, that we have received it from God's own mouth by the ministry of men, to be superior to that of any human judgment, and equal to that of an intuitive perception of God himself in it. We seek not arguments or probabilities to support our judgment, but submit our judgments and understandings as to a thing concerning which it is impossible for us to judge; and that not like some person, who are in the habit of hastily embracing what they do not understand, which displeases them as soon as they examine it, but because we feel the firmest conviction that we hold an invincible truth; nor like those unhappy men who surrender their minds captives to superstitions, but because we perceive in it the undoubted energies of the Divine power, by which we are attracted and inflamed to an understanding and voluntary obedience, but with a vigour and efficacy superior to the power of any human will or knowledge." - John Calvin, The Institutes, I. VII. V.