John Calvin Commentary Isaiah 41:21

"Produce your cause, saith Jehovah; bring forth your strong reasons, saith the King of Jacob." — Isaiah 41:21 (ASV)

Plead your cause. It was also necessary that this should be added to the former doctrine. For when we associate with wicked men, they pour ridicule on our hope and charge us with folly, as if we were too simple-minded and credulous. Our faith is attacked and frequently shaken by jeers such as the following: “These people hang on the clouds and believe things that are impossible and contrary to all reason.”

Therefore, since the Jews in their captivity would hear such mockeries, it was important that they should be fortified by these warnings of the Prophet. To give greater weight to this address, he comes forward of his own accord to inspire confidence, challenging the Gentiles themselves to bring forward everything that could support their cause, as is usually done in courts of justice.

Says the King of Jacob. When he calls himself “the King of Jacob,” he defies all idols and shows that he undertakes the cause of his people, so as to be eventually acknowledged to have vindicated his glory by delivering those who were unjustly oppressed. And yet the godly needed to possess a strong faith, for what was the aspect of the kingdom when they were captives and so severely oppressed?

This was also the reason why he formerly (Isaiah 41:14) called them “the worm Jacob” and “dead men.” But they comforted their hearts with that promise by which he formerly said that their root was concealed underground, when he compared the people to a tree that had been cut down.

A branch shall spring from the stock of Jesse, and a sprout from his roots shall yield fruit (Isaiah 11:1).

They perceived with the eyes of faith that kingly power which lay concealed, for it could not be seen by the bodily eyes or comprehended by the human understanding.