"Don't ask who will go down into the depths," (that is, to bring Christ back from the dead).
However, what else does it say? "This message is near you. It's in your mouth and in your heart." This is the message of faith that we spread.
If you declare that Jesus is Lord, and believe that God brought him back to life, you will be saved.
By believing you receive God's approval, and by declaring your faith you are saved.
Scripture says, "Whoever believes in him will not be ashamed."
There is no difference between Jews and Greeks. They all have the same Lord, who gives his riches to everyone who calls on him.
So then, "Whoever calls on the name of the Lord will be saved."
But how can people call on him if they have not believed in him? How can they believe in him if they have not heard his message? How can they hear if no one tells [the Good News]?
How can people tell the Good News if no one sends them? As Scripture says, "How beautiful are the feet of the messengers who announce the Good News."
But not everyone has believed the Good News. Isaiah asks, "Lord, who has believed our message?"
So faith comes from hearing the message, and the message that is heard is what Christ spoke.