John Calvin Commentary Isaiah 51:12

"I, even I, am he that comforteth you: who art thou, that thou art afraid of man that shall die, and of the son of man that shall be made as grass;" — Isaiah 51:12 (ASV)

I, I am. Here the Lord not only promises grace and salvation to the Jews, but remonstrates with them for refusing to believe him and for valuing his power less than they ought. It is exceedingly shameful to tremble at the threats of men to such a degree as to care nothing about God’s assistance, for he displays his power for this purpose, that he may at least fortify us against every attack. Accordingly, by an excessive fear of men we betray contempt of God.

Thus, it is evident how sinful it is to be agitated by the terrors of men when God calls us to repose. And indeed, it is amazing ingratitude in men who, when they hear that God is on their side, derive no hope from his magnificent promises, so as to venture boldly to exclaim, “If God be for us, who can be against us?” (Romans 8:31). The consequence is that when dangers arise, they are terrified and confounded, and attribute far more to the power of mortal man in attacking than to the power of God in defending. Justly, therefore, does he upbraid the Jews with not fortifying themselves by these promises and with not rendering themselves invincible against every danger; for God is treated with the highest dishonor when we doubt his truth—that is, when we are so completely overcome by human terrors that we cannot rest on his promises.

The repetition, I, I, is highly emphatic. He who promises consolation is the God of truth, against whom neither the strength nor the contrivances of men will be of any avail. When you distrust him, it follows that you do not consider who he is.

That thou shouldest be afraid of a man. He describes how frail, fading, transitory, and unsubstantial the condition of men is, in order to exhibit more fully their criminal stupidity in preferring a shadow and smoke to God. He shows that men, so long as they are mindful of God, cannot be struck down by fear. Consequently, when we are stunned by dangers that assail us, it follows that we have forgotten God; and therefore he adds—