The necessity of the preaching of the gospel. Paul rightly observes that it all goes back to the preaching of the gospel, and preachers must be sent - both by God and the Christian community at large. Obviously, the salvation Isaiah prophesied about could not be salvation through works or the law. The prophets foretold this rejection of the gospel by Israel. Because many among them had not believed his report - because they have not trusted in God s Word through Isaiah and other messengers of the gospel, they are not saved.
Hearing is a reflection of first-century life.
Paul does not raise the possibility of the message being read. While there were people who could read, the ordinary first-century citizen depended rather on being able to hear something. Yet they ignored this word also, making them more accountable.