John sees himself as standing on the threshold between the old world and the new.
In calling for repentance and baptism, he helps others to step over that threshold. He even helps Jesus over it, putting him on the right path from his quiet life in Nazareth to his public ministry in the world, with its associated political and religious tensions.
In “preparing the way for the Lord”, John also crosses the threshold. He will never be able to get back to the quiet of the desert, and neither will Jesus be able to retreat to the privacy of his old life in Nazareth. The same applies to us: once we begin to tread the path that leads of God, there is no turning back.