Mark 1:7
And preached, saying, There cometh one mightier than I after me, the latchet of whose shoes I am not worthy to stoop down and unloose. And preached, saying, There cometh one mightier than I after me, the latchet of whose shoes I am not worthy to stoop down and unloose.
The verse stands at the very beginning of the gospel account of Jesus Christ, the Son of God. There John is baptizing and preaching in the wilderness to prepare the way of the Lord. In that setting John announces that One mightier than himself is coming after him, and that coming One is Christ.
John makes the announcement in order to turn the attention of the people away from himself to Christ. He does so with deep humility, declaring himself not worthy even to stoop down and untie the latchet of the coming One's shoes. The reason for such lowliness is the exaltation of Christ's work and mission, which is so much greater than John's own that even the lowliest service to Him is too high an honor for John.
By thus pointing beyond his own baptism to the mightier One to come, the verse teaches that having received water baptism is not enough in itself. Believers need the Holy Spirit just as much as baptism, and must seek Him rather than resting satisfied upon baptism already received.