Paul is explaining the difference between being under the law versus being under grace through Christ. He uses the analogy of marriage - that believers were once "married" to the law but now through Christ's sacrifice are free to be "married" to grace. Being under the law showed Paul his sinfulness but provided no hope of salvation, while being united to the resurrected Christ through faith provides forgiveness and the ability to live a spiritual life empowered by the Holy Spirit rather than enslaved to sin. Paul describes an ongoing inner struggle between desire to do good and doing evil, which can only be overcome by relying on Christ for victory.