John replied in the words of Isaiah the prophet, "I am the voice of one calling in the desert, 'Make straight the way for the Lord.'"
This must have taken them by surprise. What could it mean but that a King was coming? The anointed one. John's preaching about the need for repentance linked with the instruction to clear a path for the coming king must have struck them with terror. They may not have understood what the Messiah was going to do, but John's universal call for repentance to Pharisee and Sadducee as well as to tax collectors and sinners must have frightened them. The Messiah was coming and John was telling them that they weren't ready.
Would we be ready for the Lord's return? He will come as a thief in the night when we are least expecting him?