Calvin teaches that the authority of Scripture does not rest on the church's decision but on the internal testimony of the Holy Spirit. The elect, whose minds have been enlightened by the Spirit, know that the Scripture is God's word.
"Scripture bears upon the face of it as clear evidence of its truth, as white and black do of their color, sweet and bitter of their taste" (Institutes I.vii.2).
To those who demand that the inspiration of the prophets be proved by reason, Calvin replies that ...
"the testimony of the Spirit is superior to reason. For as God alone can properly bear witness to his own words, so these words will not obtain full credit in the hearts of men, until they are sealed by the inward testimony of the Spirit" (I.vii.4).
Therefore, the same Spirit who inspired the prophets and the apostles to communicate God's word to us must also penetrate our hearts to seal and confirm our faith and enlighten our minds to receive the Scriptures as God's word.
"Let it therefore be held as fixed, that those who are inwardly taught by the Holy Spirit acquiesce implicitly in Scripture; that Scripture carrying its own evidence along with it, deigns not to submit to proofs and arguments, but owes the full conviction with which we ought to receive it to the testimony of the Spirit. Enlightened by him, we no longer believe, either on our own judgment or that of others, that the Scriptures are from God; but, in a way superior to human judgment, feel perfectly assured — as much so as if we beheld the divine image visibly impressed on it — that it came to us, by the instrumentality of men, from the very mouth of God. We ask not for proofs or probabilities on which to rest our judgment, but we subject our intellect and judgment to it as too transcendent for us to estimate" (I.vii.5).
Evidences, arguments, and rational proofs can only take us so far and in fact do not provide full assurance. But by the Spirit's inner testimony we arrive at a spiritual certainty that is "superior to human judgment."
"We feel a divine energy living and breathing in [the Scripture] — an energy by which we are drawn and animated to obey it, willingly indeed, and knowingly, but more vividly and effectually than could be done by human will or knowledge" (I.vii.5).
Therefore, the best way to feel this "divine energy" in Scripture and to be convinced of its divine authority is to read the profane authors and then to turn to the word of God.
"Read Demosthenes or Cicero, read Plato, Aristotle, or any other of that class: you will, I admit, feel wonderfully allured, pleased, moved, enchanted; but turn from them to the reading of the Sacred Volume, and whether you will or not, it will so affect you, so pierce your heart, so work its way into your very marrow, that, in comparison of the impression so produced, that of orators and philosophers will almost disappear; making it manifest that in the Sacred Volume there is a truth divine, a something which makes it immeasurably superior to all the gifts and graces attainable by man ... Those who feel [the works of the prophets] insipid must be absolutely devoid of taste" (I.viii.1-2).
Does this mean that evidences and arguments are worthless? Far from it, but they must be used in a subordinate role.
"These [evidences and arguments], however, cannot of themselves produce a firm faith in Scripture until our heavenly Father manifest his presence in it, and thereby secure implicit reverence for it. Then only, therefore, does Scripture suffice to give a saving knowledge of God when its certainty is founded on the inward persuasion of the Holy Spirit. Still the human testimonies which go to confirm it will not be without effect, if they are used in subordination to that chief and highest proof, as secondary helps to our weakness. But it is foolish to attempt to prove to infidels that the Scripture is the Word of God. This it cannot be known to be, except by faith" (I.viii.13).
[All quotes from the Beveridge translation.]