In the Apostle Paul's quest to explain the place of the Law in the Christian's life, he uses the ordinance of marriage as a metaphor to show difference between living under the law and living by the Spirit. So, long as a person is married, they cannot marry anyone else. In turn, as soon as the spouse dies, then the spouse that is still living is free to marry another. The Christian or person who has been born of God is no longer under the covenant of marriage by law, but has died to it and is now under the covenant of grace through Christ via the Spirit. That is not to say that the law is unholy or unrighteous. It is to stay that it only serves to expose sin for what it is, and as a moral guide, not to exact grace or salvation from God, but to teach of of our need for God's grace that is given freely.