John the Baptist prepares the way for Jesus – the lamb of God, who takes away the sin of the world. John the Baptist is sure that Jesus is the one who has come to fill us with the Holy Spirit and that he would come again in the future to deal with the wickedness of the world.
Thus, this second Sunday of Advent is about the second coming of Jesus – when he will take away the sin of the whole world.
He will become greater; I must become less.