Christians are obligated to obey the Ten Commandments, because what we find in the Ten Commandments are the laws of God. What we find in Jesus’s interpretation in the Sermon on the Mount is that the standards of the law are much higher than we had assumed. It’s not just not committing adultery and not murdering and not stealing. Jesus says, in interpreting the sixth commandment, that if you harbor bitterness, if you’re unable to forgive someone, if you call a person raca (that is, to consider him a nonperson), then you’ve murdered that person in your heart. He also says that if you lust in your heart, you’re breaking the seventh commandment and committing adultery. And you are being greedy if you’re materialistic and you’re not radically generous. So Jesus raises the bar of the commandments to the highest level.
Martin Luther wrote that you cannot break the rest of the commandments without first breaking the first one. That is, if you break the commandments, you are looking at other things as your ultimate value and your god rather than God himself.
Luther also said that when there is a negative prohibition in the Ten Commandments, a positive implication is assumed. Therefore, when it says that you ought not to murder, it also means that you ought to radically love others, even neighbors and enemies. And when it says you ought not to commit adultery, the assumption is that you’re supposed to be faithful to your wife or to your husband and to recognize sexuality as a beautiful gift from God. And therefore if you’re in a marriage relationship, you ought to recognize that it is a covenantal commitment between a man and a woman. When it says that you ought not to steal, the understanding is that you ought to be radically generous.
These are the responsibilities that Christians have in responding to the Ten Commandments. But the problem is that we’re unable to obey them perfectly. So how are we going to resolve that tension?
Jesus Christ is the second Adam, the true Israel, the individual divine corporate head and representative who has come to fulfill the obligations of the law perfectly in himself. His obedience and righteousness now gets imputed into our lives, thereby giving us the ability to obey the obligations and the demands of the law. Even when we don’t obey them perfectly, we know that we are not going to be crushed by the law, and we will have confidence as we seek to obey the law of God because we know that Jesus Christ has fulfilled those requirements perfectly for us. Therefore, we can live without fear of rejection from God for our disobedience or lack of perfect obedience. But we know that Jesus Christ has accomplished all these things, fulfilling the requirements of the law perfectly for us.