Therefore illumined by [the Spirit's] power, we believe neither by our own [note this!] nor by anyone else's judgment that Scripture is from God; but above human judgment we affirm with utter certainty (just as if we were gazing upon the majesty of God himself) that it has flowed to us from the very mouth of God by the ministry of men. (I, vii, 5) - Calvin [with Piper's comments in brackets]
This is almost baffling. He says that his conviction concerning the majesty of God in Scripture rests not in any human judgment, not even his own. What does he mean? As I have wrestled with this, the words of the apostle John have shed the most helpful light on what Calvin is trying to explain. Here are the key words from1 John 5:7-11:
And it is the Spirit who bears witness, because the Spirit is the truth. . . . If we receive the witness of men, the witness of God [= the Spirit] is greater; for the witness of God is this, that He has borne witness concerning His Son. . . . The witness is this, that God has given us eternal life, and this life is in His Son.
In other words the "witness of God," that is, the inward witness of the Spirit, is greater than any human witness - including, I think John would say in this context, the witness of our own judgment. And what is that witness of God? It is not merely a word delivered to our judgment for reflection, for then our conviction would rely on that reflection. What is it then? Verse 11 is the key: "The witness is this: that God has given us eternal life." I take that to mean that God witnesses to us of his reality and the reality of his Son and his Word by giving us life from the dead so that we come alive to his majesty and see him for who he is in his Word. In that instant we do not reason from premises to conclusions, we see that we are awake, and there is not even a prior human judgment about it to lean on. When Lazarus wakened in the tomb by the call or the "witness" of Christ, he knew without reasoning that he was alive and that this call waked him.