John Calvin on the Internal Testimony of the Holy Spirit

“The testimony of the Spirit is more excellent than all reason. For as God alone is a fit witness of himself in his word, the Word will not find acceptance in men’s hearts before it is sealed by the inward testimony of the Spirit. The same Spirit therefore who has spoken through the mouths of the prophets must penetrate into our hearts to persuade us that they faithfully proclaimed what had been divinely commanded . . . because until he illumines their minds, they ever waver among many doubts!” (Institutes, I, vii, 4)

Q. 4. How doth it appear that the Scriptures are of the Word of God?

The Scriptures manifest themselves to be the Word of God, by their majesty and purity; by the consent of all the parts, and the scope of the whole, which is to give all glory to God; by their light and power to convince and convert sinners, to comfort and build up believers unto salvation: but the Spirit of God bearing witness by and with the Scriptures in the heart of man, is alone able fully to persuade it that they are the very Word of God.

-Westminster Confession