The first reading today speaks of God as out Lord. We are used to calling God our Lord, but we do not behave ourselves like God is the lord of our lives.
The first reading tells us that we must be holy because our lord is holy. That is saying that by acknowledging someone as our lord we are binding ourselves to imitating that person. Like the reading says, “we must be holy because our lord is holy.” In the Middle Ages when people lived in total subjection to their temporal lords they advertised their need to imitate him by wearing that man’s livery.
In the Gospel Jesus is in the role of or lord who has the right to be obeyed. As our lord he commands, “I say to you, love your enemies!”
I don’t care for the way our English Bibles present Our Lord’s final words. They have him saying, “Be perfect, just as you heavenly Father is perfect.” The Greek word Matthew used there, telios, means complete or well rounded, not perfect. None of us can be perfect the way God is perfect, but we can aspire to be as well rounded, to be as kind, to be as open, and to be as industrious as our flawed nature lets us be.