We all know that stories can have more than one meaning, fairy tales for example, fairy tales usually bury within their events a moral point or lesson. In some instances the point is obvious, in others it can take time to see, but sometimes it takes someone else pointing it out to make sense. In our passage today, Paul dives into a well known historical story from Israel's past. 
In this story, the story of Abraham and his two sons, Paul finds a meaning and a lesson that both challenges the thoughts and values of his opponents while at the same time encouraging and comforting the church. It's a tricky passage, but one that presents the real challenge we face in understanding and accepting the gospel.