“John answered them, ‘I baptise with water; but standing among you - unknown to you - is the one who is coming after me; and I am not fit to undo the strap of his sandal.’” John 1:26-27
John baptised with water for repentance; the One who was to come after him was to baptise with the Holy Spirit. Only the ones who were specially chosen by God could recognise the Messiah. For this reason, John strove with all his might to bring his listeners to repentance for their sins - repentance alone would open their eyes to recognise the glory of Jesus.