In Matthew chapter 5, the first part of the Sermon on the Mount, Christ teaches what it means to truly fulfill God’s law, that it’s not just making sure God’s regulations are kept but that the heart and mind must submit as well. We might never actually commit the physical act of murder, for instance, but our heart and thoughts are so set against someone that for all intents and purposes we want them as dead as if we had actually killed them. We might never actually commit the physical act of adultery but we have thought and dreamed and fantasized it so much that for all intents and purposes it has produced the same results inside us as though we had. Christ teaches that God’s true followers fulfill His law not just physically but from the mind and heart, which results in changing their behavior and actions so that they obtain what God’s law intended – a holy (separated unto Him) and righteous (doing things His way) relationship.
Now in chapter 6, Jesus takes this thought deeper still from the issues of behavior attached to fulfilling/obeying God’s Word to bringing into submission the behaviors most associated by non-Christians as witness of Christ in us: “…practicing your righteousness…” (v.6:1)