Mark Driscoll with a helpful illustration:
My current preaching series on the Ten Commandments is expressly dealing with what is referred to as “the Law.” While most everyone would agree that at least some of the laws in the Old Testament should apply today—no one wants to be robbed and then killed, for example—the controversial question is whether or not all Old Testament laws are binding on Christians today.Further on...
This issue is timely, as it has become popular to mock Old Testament laws as outdated, for example, whenever the issue of homosexuality arises in the media. The common way this plays out on television talk shows is simple: The Christian says that the Bible forbids homosexuality. The non-Christian then says that the Bible also forbids eating crab and wearing a poly-cotton blend shirt. Everyone laughs, the Christian loses, and the audience goes home to watch porn.
...perhaps a simple illustration will help. In high school there are a lot of rules/laws/requirements that I needed to abide by in order to get an acceptable report card and graduate. Once graduated, some of the laws at my school remained binding upon me while others were fulfilled and are no longer binding. For example, in my school I was not supposed to stab people, blow up public property, or pull fire alarms. Even though I have graduated, those laws remain binding upon me for the rest of my life.
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There were other laws, however, that are no longer binding because the requirements were fulfilled. For example, I had to show up a certain number of days for a certain number of hours, I had to sit in class, and I had to take tests and pass them. Now that I have graduated, I no longer have to do any of those things because the demands of the law were fulfilled, which culminated in my graduation.
Similarly, Jesus went to school for me. He got straight A’s in life without ever sinning. He passed every moral test he took. He met every requirement of the law. And, by grace he is my report card. He has given me his GPA/righteousness and because of that I have graduated from the law and am no longer bound by its demands as I once was (Gal. 3:24–26).
However, Jesus is also my Lord and he has chosen some moral laws to remain binding upon me for his glory, and for other's good. So, I can eat shellfish while wearing a poly-cotton blend but I still cannot murder my neighbor or steal from my employer. Jesus wants me to obey these laws not for my righteousness, but from my righteousness in him and out of love for him and my neighbor.