Charles Ellicott Commentary Isaiah 7:9

"and the head of Ephraim is Samaria, and the head of Samaria is Remaliah`s son. If ye will not believe, surely ye shall not be established." — Isaiah 7:9 (ASV)

If you 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 his look, or possibly by his words, he showed his disbelief, and Isaiah offers to meet it, in the awareness 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 especially characteristic of Isaiah. (Isaiah 38:7.) There is something significant in the Lord your 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 awareness of his mission or may imply some previous relationship with the young king as a counselor and teacher. (See Introduction.)