God's Great and Vast Mercy
Matthews 20:1-16 serves as a reminder for us about how great and vast God's mercy and generosity are. Our salvation is not earned, but freely given by God.
Jesus provides salvation for all who accept His invitation, no matter the timing. Sometimes, we can be like the laborers who worked the fields all day, and think we are a "better Christian" somehow based on how long we have faithfully practiced our faith, how much we pray, or volunteer, or give to charity. Jesus wants us to focus on the fact that we should be thankful that we have been invited to "labor in the field" at all.
Christ invites us to focus less on comparing ourselves to others and more on our interior life; our personal relationship with God. Only when we strive to love God with all our heart, mind, and soul can we truly love our neighbor as ourselves.