Calvin on why we are certain the Bible is true.

Calvin insisted that the testimony of the Holy Spirit is superior to reason. It is an inward work that captures the minds of those who hear or read Scripture, producing conviction or certainty that it is the Word of God with which they are dealing. This is a second work of the Holy Spirit with respect to the Scriptures. He who had originally inspired the prophets and apostles to write the Scriptures now penetrates our hearts, convincing us that these Scriptures are indeed the Word of God and thus truth.

Erickson, M J 1998, Christian Theology (2nd Ed), page 869