Isaiah 7:9 - Ellicott's Commentary On The Whole Bible

If ye will not believe... — The prophet reads the thoughts that were working in the king’s mind. He had no faith in these predictions terminating at a date which he was not likely to live to witness. By look, or possibly by words, he showed his incredulity, and Isaiah offers to meet it, in the consciousness of a Divine power that will not fail him. From Heaven to Hades, Ahaz may take his choice. The method of giving a sign by predicting something in the near future as a pledge for predictions that belong to a more remote time is specially characteristic of Isaiah. (Comp. Isaiah 37:30; Isaiah 38:7.) There is something significant in “the Lord thy God.” Ahaz, idolater as he was, had not formally abandoned the worship of Jehovah. The tone of authority in which Isaiah speaks may be either that belonging to his consciousness of his mission, or may imply some previous relation to the young king as a counsellor and teacher. (See Introduction.)