After describing the picture of slavery to sin compared to slavery to God, Paul is now going to use the picture of marriage specifically regarding the Jews. When they left Egypt, they were married to Christ in Exodus 24 when they swore that they would do whatever He told them to do and that marriage was governed by the law, whatever You say, we will do. Because it was governed by the law, for it to go away, they must die to marry another. According to the law of marriage, without committing sin worthy of death, the only way out of a marriage is through death. In death, that marriage is done away with and you have no responsibility to do what your dead husband commanded. For them to escape the curse which just produced sin, whatever You say we will do, is to die, then that relationship ends. To be saved, we must first stop thinking that whatever God says, we will do because that is works based righteousness and still being under that relationship means you are still condemned. You must die to your relationship with the law, so that you can be in the Spirit not just the Jews but also the Gentiles.