The Deutero-Isaian part of the book describes how God will make Jerusalem the centre of his worldwide rule through a royal saviour (a messiah) who will destroy her oppressor (Babylon); this messiah is the Persian king Cyrus the Great, who is merely the agent who brings about Yahweh's kingship. Isaiah speaks out against corrupt leaders and for the disadvantaged, and roots righteousness in God's holiness rather than in Israel's covenant. Isaiah 44:6 contains the first clear statement of monotheism: "I am the first and I am the last; besides me there is no God". This model of monotheism became the defining characteristic of post-Exilic Judaism, and the basis for Christianity and Islam.
Isaiah was one of the most popular works among Jews in the Second Temple period (c. 515 BCE – 70 CE). In Christian circles, it was held in such high regard as to be called "the Fifth Gospel", and its influence extends beyond Christianity to English literature and to Western culture in general, from the libretto of Handel's Messiah to a host of such everyday phrases as "swords into ploughshares" and "voice in the wilderness".
The scholarly consensus which held sway through most of the 20th century saw three separate collections of oracles in the book of Isaiah. A typical outline based on this understanding of the book sees its underlying structure in terms of the identification of historical figures who might have been their authors:
While one part of the consensus still holds – virtually no contemporary scholar maintains that the entire book, or even most of it, was written by one person – this perception of Isaiah as made up of three rather distinct sections underwent a radical challenge in the last quarter of the 20th century. The newer approach looks at the book in terms of its literary and formal characteristics, rather than authors, and sees in it a two-part structure divided between chapters 33 and 34: