Krister Stendhal, and others, assert that the climax of the letter is in chapters 9 through 11 in its discussion of the redemption of the Gentiles and the salvation of Israel (from Reinventing Paul, John G. Gager).
Paul preaches that Christians do not have to become Jews to be included in God's family. Nor do Jews have to become Christians in order to stay:
to them belong the adoption, the glory, the covenants, the giving of the law, the worship, and the promises; to them belong the patriarchs, and from them, according to the flesh, comes the Messiah, who is over all, God blessed forever (Romans 9:4-5).
The question for us moderns is whether God still has flexibility in defining family.