How, then, can they call on the one they have not believed in? And how can they believe in the one of whom they have not heard? And how can they hear without someone preaching to them? And how can they preach unless they are sent? As it is written, "How beautiful are the feet of those who bring good news!" Rom 10:14,15As an evangelist himself called by the Lord Jesus Christ to preach the gospel, what stood out to Paul in this verse was the fact that the message had to be brought by someone who had been sent to preach it. So also it had to be preached and so to be saved God made it such that people must be willing first of all to go and preach it, and people must be willing to listen.
This process can break down on two levels. For God's people must be willing and prepared to preach, as it is written, "having shod your feet with the preparation of the gospel of peace" Ephesians 6:15 We are called to embrace the concept of mission in our living of the Christian life as Jesus said, "go and make disciples of all nations" Matt 28:19
But even if one goes, another problem is one of persistence. For there will be much opposition as Paul can testify along with all the apostles and prophets, Jesus Christ himself being our example. "Consider him who endured such opposition from sinful men, so that you will not grow weary and lose heart." Hebrews 12:3 If people aren't willing to listen then they cannot be saved. This has become a particularly difficult roadblock in Islamic countries where millions will go to hell because either they personally won't listened, but more significantly their culture itself disallows the preaching of the gospel.
Pray therefore not only for the Lord of the Harvest to raise up laborers for the harvest, but also that roadways may be opened to allow the communication of the good news for those who otherwise will have had no opportunity to hear.