Notice vv.17-21, “But if, while we seek to be justified by Christ, we ourselves also are found sinners, is therefore Christ the minister of sin? God forbid. For if I build again the things which I destroyed, I make myself a transgressor. For I through the law am dead to the law, that I might live unto God. I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain.” The bottom line is that righteousness does not come through keeping the Law of Moses. As a matter of fact, it is impossible to keep the Law of Moses. You and I simply cannot do it.
Now, some may be sitting there right now wagging their self-righteous heads and thinking to themselves, “I can keep the Law of God.” The Pharisees of Jesus’ day made the same mistake. They thought that as long as they physically kept the Law that they were okay. For example, if the Law says, “Thou shalt no commit adultery”; that meant that as long as they didn’t physically commit adultery they were okay. However, “window shopping” was okay. Then Jesus comes along and blows that view totally out of the water when he says, “But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart” (Matthew 5:28). That changed things considerably because who hasn’t looked with lustful eyes at another person?
That’s also why Jesus said in Matthew 5:20, “For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.” What Jesus was saying is that the righteousness of the Pharisees was just external while what they really needed was an internal righteousness. In other words, just keeping the Law externally (not physically committing adultery) while breaking the Law internally (lusting after others in our hearts) was still breaking the Law and thus, Heaven was not an option!