Isaiah 40:28

Hast thou not known? -This is the language of the prophet reproving them for complaining of being forsaken and assuring them that God was faithful to his promises. This argument of the prophet, which continues to the close of the chapter, comprises the main scope of the chapter, which is to induce them to put confidence in God, and to believe that he was able and willing to deliver them. The phrase, ‘Hast thou not known? refers to the fact that the Jewish people had had an abundant opportunity of learning, in their history, and from their fathers, the true character of God, and his entire ability to save them.