Why subtitle a commentary on Romans The Divine Marriage? Mainly because the central message of the Bible has to do with the drama of God seeking out a people for himself. The Old Testament described Israel as God's bride because she was called to a unique, personal relationship with her God.
However, Paul's contention is that national Israel's exclusive claim to be the bride no longer stands. The apostle's message is that God has created a new covenant with those who believe in his Son, and that believing Jews and Gentiles have now become the true bride of God. The Jewish remnant and believing Gentiles both draw from the same divinely-appointed stock as they share the promises given by God to Abraham.
The theme of the divine marriage (which is the culmination of the New Exodus) shaped and guided the letters that Paul wrote. This is especially true for the letter to the Romans, the letter of the divine marriage.