John Calvin Commentary Isaiah 40:28

"Hast thou not known? hast thou not heard? The everlasting God, Jehovah, the Creator of the ends of the earth, fainteth not, neither is weary; there is no searching of his understanding." — Isaiah 40:28 (ASV)

Have you not known? He repeats the same statement which he had previously made, that the people who had been carefully taught in the school of God were inexcusable for their slothfulness, and sharply rebukes them for not having profited more by the doctrine of the Law, and by the other means which God had bestowed in addition to that knowledge which they possessed in common with the Gentiles. The word know, which is more general, is put first, because by many miracles and other proofs God had manifested his glory. Next, he asks, Have you not heard? As if he had said, “If you have profited nothing by being taught by actions and by word that God is never unemployed, it is evident that you are excessively unteachable.”

That Jehovah is the God of eternity. The Prophet calls him “eternal,” and thus distinguishes him from all idols, which endure but for a time, and were made by men. Truly, if this were deeply seated in our hearts, there would no longer be any room for distrust. For if God is eternal, he never changes or decays, eternity being uniformly attended by this quality: that it is never liable to change, but always remains the same. Since the Jews did not sufficiently believe these things, though they had often heard them, the Prophet intended to arouse them by this rebuke, in order to show that they will be doubly guilty before God if, after having been taught both by his numerous benefits and by the word, they do not render the honor and glory which are due to him.

And is not wearied by weariness, and there is no searching of his understanding. Here the Prophet makes two statements: first, that God is not wearied in doing good; and secondly, that no man can explore his wisdom. In the former clause he shows that nothing will hinder God from continuing to exercise his kindness, for he is not like men whose resources are exhausted by giving frequently, or who are wearied by continually bestowing new favors, or who repent of their generosity.

His kindness is never exhausted. If he was kind to the fathers, he will be no less kind and bountiful to posterity.

As to the allegation that God very often acts differently from what we think to be best for us, the Prophet addresses this by saying that his purpose is incomprehensible. He also warns us that we should not murmur, though he does not comply with our wishes all at once. For nothing is better adapted to cherish our hope than this sobriety, which leads us to consider how marvelously God works in preserving us, and thus to submit to his secret counsel.