"a married woman is bound by law to her husband while he lives, but if her husband dies she is released from the law of marriage. Accordingly, she will be called an adulteress if she lives with another man while her husband is alive. But if her husband dies, she is free from that law, and if she marries another man she is not an adulteress.
Likewise, my brothers, you also have died to the law through the body of Christ, so that you may belong to another, to him who has been raised from the dead, in order that we may bear fruit for God." (Romans 7:2-4)
Cast of characters:
Woman = Paul's brothers, Israel (and the Gentile church grafted on) [thanks Chris]
First husband = The first Adam, the old humanity, Jesus Christ "in the likeness of sinful flesh" (Rom 8:3)
Second husband = The last Adam, the new humanity, Jesus Christ in his "spiritual body" (1 Cor 15:44)
Law = marriage covenant between Israel and the first Adam, the bond that unites
Spirit = marriage covenant between the church and the last Adam (Rom 7:6), the bond that unites
The law was the equivalent of a marriage certificate binding us to Adam. "All that was his, is yours" it declares. All his death and all his sin.
But Adam died on the cross, and if we believe that then we can get remarried without breaking the law. If we don't believe that then we'll keep living in the house of death instead of moving in with our new husband into the house of life which is ours by faith.
The Spirit promises us that we are united to the risen Christ in marriage. "All that was his, is yours" it declares. All his life and all his righteousness.