The Spirit and the Word

But I answer, 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. The same Spirit, therefore, who spoke by the mouth of the prophets, must penetrate our hearts, in order to convince us that they faithfully delivered the message with which they were divinely entrusted. This connection is most aptly expressed by Isaiah in these words, “My Spirit that is upon thee, and my words which I have put in thy mouth, shall not depart out of thy mouth, nor out of the mouth of thy seed, nor out of the mouth of thy seed’s seed, saith the Lord, from henceforth and for ever,” (Isa. 59:21). Some worthy persons feel disconcerted, because, while the wicked murmur with impunity at the Word of God, they have not a clear proof at hand to silence them, forgetting that the Spirit is called an earnest and seal to confirm the faith of the godly, for this very reason, that, until he enlightens their minds, they are tossed to and fro in a sea of doubts.

John Calvin, Institutes of Christian Religion, 1.7.4.