Charles Ellicott 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? ... —The questions are parallel to those of Isaiah 40:21, but are addressed to the Israel of God, rather than, as those were, to mankind.

The Creator of the ends of the earth. —The word emphasizes the thought that the whole earth, from the Euphrates to the “islands” of the sea, is subject to the power of the Eternal.

Faints not, neither is weary? ... —Had Isaiah learned to feel that even his own phrase concerning men “wearying God” (Isaiah 7:13) was too boldly anthropomorphic, and might, therefore, be misleading?

No searching of his understanding. —The words come, like so many others like it, from Job (Isaiah 5:9; Isaiah 9:10), and must have been in St. Paul's mind as he wrote Romans 11:33.