The book of Isaiah marks the beginning of the so-called major prophets, a division that also takes in Jeremiah, Ezekiel, and Daniel.
The minor prophets (minor merely because they are shorter) follow, from Hosea to Malachi. With the failure of the priesthood (1 Sam. 1-4), the prophet, by God’s sovereign appointment, became the means whereby He could address Himself to the conscience of His people. The priest was the people’s representative before Jehovah; the prophet was Jehovah’s mouthpiece to the people.
Prophecy presents the mind of God, not just with respect to future events, but also as to present state. It is for this reason that the books of Joshua through Kings are to be found in the Hebrew Scriptures within the portion known as “The Prophets” (Luke 24:4444And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me. (Luke 24:44)).
Prophecy is two-sided. On the one hand, it reveals the sinful state of the people and God’s judgment—His strange work (Isa. 28:2121For the Lord shall rise up as in mount Perazim, he shall be wroth as in the valley of Gibeon, that he may do his work, his strange work; and bring to pass his act, his strange act. (Isaiah 28:21)). On the other hand, it reveals His heart in love, in particular the promise of the coming of the Messiah. One cannot be without the other.