A lot of the writings of St. Paul were generated as responses to comments and letters from those he knew. Also, he answered the refutations of his enemies. One of the greatest refutations of Christianity in Paul's day, which was continually thrown into the faces of the Christians by the unbelieving Jews, was...
"If Jesus was truely the Messiah then why wasn't all of Israel saved?" (As opposed to just the sect of the Christians)
Paul wrote Romans in the winter of 57AD at Corinth. He is answering this charge.
Romans 9:2-9 (NASB)
...that I have great sorrow and unceasing grief in my heart. [3] For I could wish that I myself were accursed, separated from Christ for the sake of my brethren, my kinsmen according to the flesh, [4] who are Israelites, to whom belongs the adoption as sons and the glory and the covenants and the giving of the Law and the temple service and the promises, [5] whose are the fathers, and from whom is the Christ according to the flesh, who is over all, God blessed forever. Amen.
[6] But it is not as though the word of God has failed. For they are not all Israel who are descended from Israel; [7] neither are they all children because they are Abraham's descendants, but: "through Isaac your descendants will be named." [8] That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants. [9] For this is a word of promise: "At this time I will come, and Sarah shall have a son."
And so he is saying that all of Israel was saved because it is the children of promise and not genetics that makes you a citizen of the true Israel. Thus he answers the charge that Jesus was an impostor.