Today's liturgical readings match marvelously: St. Paul beginning his letter to "all in Rome, called to holiness...." and Jesus commenting on the lack of faith of those around him who were asking for a "sign from heaven." The only sign they will get, he says, is the Sign of Jonah. And that leads into a little set of comparisons of faith: The Queen of the South who welcomed the wisdom of King Solomon gets praise that "this generation" does not win from Jesus. And the men (he specifies "men") of Nineveh who heard the Word of God preached by Jonah.
When this Gospel is used in the liturgy, it is cut off from the passage that directly precedes it (which was Saturday's Gospel): the woman in the crowd who praised Jesus' mother for her physical childbearing and nuturing, and who was told "blest are those who hear the word of God and keep it." Taken together, the section of Luke is really about what we do when the Word of God comes to us. Mary, the Queen of the South and the men of Nineveh all welcomed a surprising message that came to them unexpectedly. They did not hold out for "signs from heaven" but heard the Word of God and kept it.