The prophet seems particularly concerned with monotheism: That there is only one God, and all idols are not gods at all, are in fact nothing more than the material they are carved from. This would be an obvious reaction to spending 70 years living among the Babylonian polytheists, but might feel a bit out of place for someone living in Jerusalem prior to the 8th c. exile.
Isaiah, however, caricatures idolatry and attacks a straw dog. The typical practice of idol worship did not presume the hand-made idol to actually be a god. Rather, the people formed images to represent one or more of the various gods they believed to exists, then through complex rituals hoped to entice the essence or spirit of that god into the idol, where it would be able to hear the prayers and petitions of the worshiper. In fact, idol worship worked much like the temple did for the Israelites, in that Israel believed the presence of their God could be accessed only at the temple in Jerusalem (which they did indeed build with their hands), while typical 7th c. BCE idolaters believed they could communicate with their gods via these portable handmade idols.
Much of Isaiah’s proof that God is the one true God rests on His ability to predict the future. If the prophets of old made incorrect prophecies, how does that bode for the reliability of God?