The parable in the Gospel illustrates two different kinds of people in their relationship with God. The Jewish leaders are those who said they would obey God but did not. The tax collectors and 'sinners' though they did not intend to follow God's way but eventually they repented and obeyed God's will.
This parable reminds us of our Christian responsibility in our total submission to God's will. This presupposes that our relationship with God is that of Father/son/ daughter - a relationship of total dependence on God that moves us to obey and carry out God's will as best as we can possibly do.
In other words, we are to 'walk the talk' thus bearing witnesses of Jesus in our daily life.
The first reading spells out the ways how Yahweh/ God will bring victory over sin (Zeph. 3: 13). We only have to trust God for He would take care of everything. This also includes our submission to God's will.
Let this Advent season help us become aware of God's presence in our lives and to depend totally on this relationship.
"Lord, grant us the grace to know and do Your will always."