In John 1:24,25 The pharisees asked John the Baptist why he baptised if he was not the Messiah, Elijah or the Prophet. Why did they associate baptism with these 3 figures?
When people saw John preaching and baptizing and that thousands came to hear him, they began to wonder who he was. Might he be the fulfillment of one of their expectations? If you read the Old Testament, you will find prophecies of a coming Messiah (Daniel 9:24 and many others), of a coming prophet (Deuteronomy 18:15f and others) and of Elijah coming back as well (Malachi 4:4-6). The people who heard John may not have been very sophisiticated, but they knew that John was unique. They had not had a prophet for over four hundred years. The fact that John was baptising was significant to them, because it was not something that their rabbis did. This was not the only reason they thought he might be the Messiah, the Prophet or Elijah, but it was part of why they began to wonder if he might be the fulfillment of one of their prophecies.