How are you assured that Scripture is the Word of God and that it is truly from heaven? It is not only because Holy Scripture says this about itself, “All Scripture is divinely inspired” (2 Tim. 3:16). “Prophecy did not come about in former times by the will of man, but it is by the movement of the Holy Spirit that holy men of God spoke” (2 Pet. 1:21). We also believe this because Scripture contains such evident marks of its divine inspiration that whoever pays attention to them will find himself powerfully convinced and persuaded of this divine inspiration. But ultimately we believe because the Holy Spirit clearly testifies to it in our hearts. “The Spirit (who speaks in the heart of believers) is the one who witnesses (also in my heart) that the Spirit (who speaks in Scripture) is the truth” (1 Jn. 5:6).
HT: The Heidelblog