Jesus was telling a story about rebellious tenant farmers who had no respect for the owner of the vineyard or his representatives or his son (Luke 20:9-19).  Indeed, they decided to kill the son, assuming that they would then inherit the property as sitting tenants.  So Jesus challenged His listeners to decide what would be the right action for the vineyard owner to take.  Clearly, those who had been left in charge of the farm had demonstrated their untrustworthiness.  The only sensible conclusion would be for the owner to dismiss the farm managers and employ others who could be trusted.
 
The people knew from the Old Testament metaphors that the vineyard was Israel and that the owner was God (Jeremiah 12:10).  Whether or not they recognised Jesus as being God the Son, it was clear that Jesus did (John 10:30-33).  Speaking just a few days before His crucifixion, He accurately predicted His own death.  And He also accurately predicted that God's exclusive favour towards His chosen people would end, and that others (Gentiles), could be allowed the full rights of being in God's family because of Jesus (Ephesians 3:2-6).  

This was important teaching for the trainee apostles.  It needed to come from Jesus’ lips as they would wrestle with the idea of Gentiles being included in God’s kingdom (Acts 10:28-29; Acts 11:1-18).  However, those who listened at the time understood enough about the story to be shocked at the thought that God would dismiss Israel from its unique relationship with God and allow the Gentiles to enter His kingdom.  We all like to have exclusive privileges.  That is why many people join clubs, shopping discount schemes and foster family relationships.  For many, their religion is a special sort of club. Every organisation has conditions of entry and rules for membership, and religious people specialise in obeying the rules. That way they do not prejudice the apparent security which their membership brings.  

However, following Christ and trusting Him is not that sort of relationship.  It does not depend upon our goodness or pious observance - but on believing and receiving His grace (Ephesians 2:8-9).  So, think for a moment about your friends and colleagues who are caught up in Christ-less religion which can never satisfy God's holiness.  All such piety will mean nothing when Jesus Christ returns, but those who have come to Him in repentance and faith will never be turned away or cast out (John 6:37).  Paradoxically, it is only through Christ's death that the blessings can come to the Gentiles ... and not just the Gentiles, for also every Jew who believes will also be welcomed and received as a child of God.