The new and better New Covenant given to us through the death and resurrection of Jesus Christ freed us from Old Covenant Law keeping. Paul put it this way: "You are not under law, but under grace".
Christians are under Jesus Christ and his law. We are not under the Law of Moses because God made it obsolete (Heb 8:13). In the Sermon on the Mount Jesus contrasted the higher standard of his law with the Old Covenant. He specifically compared his law with the Ten Commandments with his "You have heard it said" statements.