The purpose of the book of Isaiah demonstrates the trustworthiness of the Lord. The first king whom Isaiah serves, Ahaz, does not trust the Lord. He ignores Isaiah’s advice and follows his own schemes. This leads to defeat and servitude at the hands of the Assyrians. Ahaz’s son Hezekiah, in contrast, trusts the Lord and Jerusalem is delivered from Sennacherib and the Assyrians. In the second half of the book the exiles are also encouraged to trust the Lord to bring deliverance and to respond like Hezekiah, not like Ahaz.
A significant theme is the hope in a future ideal Davidic king. The book provides a template for Messianic expectation as it develops a profile of God’s plan, including the exaltation of Jerusalem (ch. 2), the coming child whom is the reign (ch. 9), peace and stability of the reign of the Davidic heir(ch. 11), and how the ideal Servant of the Lord will carry out God’s mission (chs. 42-53).
Walton, Strauss, and Cooper. The Essential Bible Companion.