The confrontation between Jesus and the religious elite leads to Jesus showing in sharp contrast the difference between "the religion of the scribes and Pharisees and life in the Kingdom of God." 1-5 -- repentance is not about "feeling sorry" but is about change. Unless we make the change to the Kingdom of God we will have gained nothing. Religion gives us the illusion that we are in control -- if I say the right words, do the right actions and avoid certain bad actions I will be OK -- but that kind of control is an illusion and misses the central Biblical point that God's first and foremost desire is that we live our lives in relationship with him. The door is narrow (22ff) not because God has made it difficult but because we want to be in control.
Notice Jesus calls those on the outside "evildoers" (27). There is a difference between "doing evil" and "evil doing". Doing evil would be murder and mayhem, breaking the commandments, etc. Evil doing is doing right things from wrong motivation. Performing acts of a charity for the human accolades; serving in mission for the thrill or so we can appear to be important and "big" in our own eyes or the eyes of the world. Evil doing -- see Matthew 5-7 -- is pretty common and completely misses the point of the Gospel. The point is simple: God desires that we live our lives in a love relationship with him. This love relationship will lead us to align our wills and our desires with his which will lead us to do acts of charity and mission. The doing is a consequence of the relationship. We go because we are called and desire to go . . . not be cause we have to.
The mustard seed (18) and the yeast (20) teach us that it does not take much of the Kingdom of God in us to transform us. The smallest of seeds grows into a tree the smallest bit of yeast transforms the entire lump of dough. So it is with us -- the smallest of steps the simplest of choices the tiniest bit of change -- begins a process of entire transformation. All too often all we need to do is to make the first step and begin the journey.