Well, do we?
To say that we do would be wrong. If I am still in any way in relationship to the law, then I am condemned by it and justly so.
But I, and many Christians have believed a widespread theology that because we have received the Spirit of God, He then enables us to keep the law. Nothing could be further from the truth! If I were still under the law, I could not be indwelt by the Spirit! But where there is no law, there is no transgression.
Malcolm Smith gave a great analogy as to how silly it is to put a Christian back under the law after being saved by grace. I don't remember the exact words he used, but I do remember the gist of the story.
Malcolm, growing up in the town he was from, didn't have electricity until he was a bit older, so they were forced to light their homes by candle light at night. But as things progressed little by little, the town he lived in eventually got electricity. Then one day, being at an elderly man's house, (who belonged to Malcolm's Church) he noticed the old man nodded toward his wife, telling her to turn on the electric lighting since it was beginning to get dark outside. And then after she turned on the lights, he began to light the candles that they had used before they got electricity, saying that it would be much easier to light the candles now that they had electricity.
The Old Covenant is said in the Bible to have had glory; it had a light. But now, Paul says, the glory of the New Covenant of grace much more surpasses the glory that the Old Covenant of law had. It would be silly to go back to the law (the Ten Commandments) now that you have God on the inside of you. You have the fulfillment of the law living on the inside of you! Why now do you retreat back to the law, believing it has some work yet to finish that God has not already finished?
The works that were to be done in order for God to be pleased with you were done by Jesus Christ. Jesus Christ fulfilled all the law and the prophets, and through faith, that same righteousness that He accomplished is now fulfilled in you.