John Calvin Commentary Isaiah 45:5

"I am Jehovah, and there is none else; besides me there is no God. I will gird thee, though thou hast not known me;" — Isaiah 45:5 (ASV)

I am Jehovah. He confirms the preceding statement, and the repetition is not superfluous. It was fitting for it to be often repeated to Cyrus—that there is one God, by Whose hands all rulers and nations are governed—so that he might be drawn away from all delusions and be converted to the God of Israel. Furthermore, it is clearly stated that we should not try to find divinity in any other. It is as if He had said, “Beware of ascribing this victory to idols, or forming any confused idea of a god such as people imagine; know that the God of Israel is the only author of this victory.”

Although Cyrus did not benefit from this admonition to such an extent that he left his idols and devoted himself to the true God, it still made so deep an impression on his heart that he acknowledged Jehovah to be God and to possess the highest authority. At the same time, it was fitting that those who were members of the Church should embrace this doctrine, so that they might boldly despise all pretended gods.

I have girded you. This girding corresponds to the nakedness which He formerly mentioned (verse 1), when He said that He “opened” or “ungirded the loins of kings.” For He is said to “gird” those whom He supplies with strength and courage and renders victorious.

From this, it should be inferred that people have no courage except when the Lord imparts His power and strength to them, that neither weapons nor any military force can do anything unless He assists, and, in a word, that He presides over all wars and gives victory to whomever He pleases, that no one may think it happens by chance.

He again repeats, Though you have not known Me, to make it even more certain that these things are granted to Cyrus for the sake of the Church, so that he may show evidence that he remembers it with gratitude and may show kindness to the people of God in return for such a distinguished favor.