1 John 2:29 - 3:6 / John 1:29-34
There is a deep relationship between parents and their children.
Besides the genetic connection, children bear a physical resemblance to the parents.
That's why there are such sayings as "a chip off the old block" or "like father, like son".
John the Baptist calls Jesus the Lamb of God. The Lamb of God takes away the sins of the world by being sacrificed for sinners.
Jesus sacrificed Himself for us so that freed from sin, we can be re-formed and re-created into His image and likeness.
Hence the 1st reading says that we must purify ourselves and try to be as pure as Christ. That is because sin is so serious and devastating.
Because to sin means that we do not know Christ and we have broken off the relationship with Him and we have deformed ourselves into the darkness of sin.
But by our baptism, we have become children of God and we have become one with Christ.
In our prayer, let us renounce our sinfulness and profess our faith and our love for God, so that like John the Baptist, we too will be witnesses of the Lamb of God.