Significantly, the role of faith or belief is crucial in this passage. For people will only call on one whom they believe. Often when we say to people God can help them and they look at you in disbelief. Why? They have yet to believe in this God and how can they call on him whom they have not believed. Paul goes on to say that the people need to hear first before they can believe which means the Gospel had to be articulated verbally to convey God's message of salvation to mankind. With this aurality, the Spirit works to convict hearts until some will believe in him.
Paul goes on to argue that people will only listen if there are those who preach the Gospel. Hence, the crucial bridge between faith and hearing the Gospel is actually a human instrument, the preacher. Note that it is in plural, those who preach the Gospel following the Isaianic verse on how beautiful are the feet of those preach the good news. Interestingly, Paul does not think everyone will believe, perhaps only few will believe by citing another Isaiah quotation, "who will believe our report?", with the rhetorical question giving a sense of discouragement and doom for few will find the good news good for them.
Paul continues by linking belief to obedience where believing the gospel is not merely mental assent or intellectual apprehension but involves the whole being in obeying, namely in repentance, making amends for wrongs done, restitution, reconciliations of broken relationships and peace among former enemies.
And preachers can't preach unless they are sent? This is a theological crux for this passage. First, it speaks of authorization of the preacher, a divine calling and appointment to preach the Gospel. In fact, the preacher's authoriy is relative to the authority which appointed him as a sent one is totally dependent on the Sender. The preacher's message is not his own but the Sender's. The preacher has no right to add to or subtract from the message. He has to deliver it as he is entrusted, no more and no less. Since God is the sender, there ought to be no doubt or ambiguity to the message. It is not one's own interpretation as God is able to prepare his messengers fit for the task.
Finally, "those who are sent" could point to the possibility that it is the church that acts as the Sender of the preacher. On one hand, it is a divine appointment and sending but on the other hand, it is also a human sending since the church needs to divine God's will in the choice of preachers and missionaries like what we see in Acts 13. More often than not, modern churches get appointments wrong or leaders in wrong positions because many churches no longer fast and pray constantly until they hear what the Spirit is saying, "Set apart for me, Barnabas and Saul..."