Luke 10:26 - Albert Barnes' Notes on the Bible

What is written ... - Jesus referred him to the “law” as a safe rule, and asked him what was said there. The lawyer was doubtless endeavoring to justify himself by obeying the law. He trusted to his own works. To bring him off from that ground - to make him feel that it was an unsafe foundation, Jesus showed him what the law “required,” and thus showed him that he needed a better righteousness than his own. This is the proper use of the law. By comparing ourselves with “that” we see our own defects, and are thus prepared to welcome a better righteousness than our own - that of the Lord Jesus Christ. Thus the law becomes a schoolmaster to lead us to him, Galatians 3:24.