The Lord keeps His promises and the book of Isaiah is proof of this.  As the Lord established His covenants with Abraham and his offspring, He expected them to remain faithful to Him as He would be to them.  Isaiah is about the consequences of breaking this covenant relationship and the Lord’s gracious response to provide a solution.  This solution was the Suffering Servant who throughout his prophecy Isaiah predicts would come.  To understand the gospel, one needs to understand Isaiah.