Most of you know this but it bears remembering that chapter and verse designations are man-made. They were placed throughout the text of the Bible as a means to navigate through the Bible. In many cases they do make a clean break in the thoughts of the authors. In the Sermon on the Mount however, this is not the case.
Chapter 6 continues the explanation of Jesus on what constitutes true righteousness. He has contrasted true righteousness from false righteousness by using the religious leaders of His day.
Jesus makes the point in this chapter that what people do is a direct result of the righteousness they possess – whether their own, or the righteousness God gives us through faith in Jesus Christ.
That was certainly the intent behind what He said in 5:17-20. Up till that point Jesus had taught the people how godly righteousness was first of all apprehended and secondly how it was manifested. In that passage verse 20 is a pivotal verse in helping us understand exactly what Jesus was saying.