In today's Gospel reading [John 1:6-8,19-2], priests [in the line of Aaron] and Levites [i.e., of the tribe of animal sacrifice ministers in the temple] come to John the Baptist [who is preaching a baptism of repentance in the River Jordan] to ask him "who" questions. He was not sent by them, but by God, so their "who" questions are aimed at how he is connected with scriptural prophesy. Each "who" question asks whether he is someone identified in that prophesy as one who is to come. John answers the key question on their mind, though the Gospel does not specifically record their asking it...
"Are you the Messiah? The one spoken of by the great prophets?" Our first reading, from Isaiah, chapter 61, is such a prophesy, one which Jesus cites as being fulfilled.
John answers no.
"Are you Elijah?" The great prophet of 1st and 2nd Kings, who performed great miracles. There is a hint, in 2nd Kings, chapter 1, that John might be Elijah returned [as Malachi 3:23-24 prophesies]. That hint is found in John's garb of camel's hair and a leather belt, the very garb worn by Elijah [2nd Kings 1:8].
Again, John answers no.
"Are you the Prophet?" Notice the capital 'P'. They're not asking whether he is a prophet, but whether he is "the Prophet". Who is this special prophet? These priests and Levites are most likely asking about the Prophet mentioned by Moses in Deuteronomy 18:15-18. This Prophet will speak on God's behalf, predicting the future and telling them about God's new commandments. In other words, he will not just be a law giver. He will also be a law giver. In other words, he will be, like Moses, a prophet, law giver and judge.
Again, John answers no. Three strikes! No hits! Finally, they ask an open question.
"Who are you, so we can give an answer to those who sent us? What do you have to say for yourself?"
John answers, citing the very passage from Isaiah that we read last Sunday, Isaiah 40.
"I am the voice of one crying out in the desert,
'make straight the way of the Lord,'"
Now the priests are out of questions. They don't want to admit that they don't see what Isaiah was getting at. The Pharisees, however, think of something else to ask.
"Why then do you baptize if you are not the Christ or Elijah or the Prophet?"
John's answer clarifies what Isaiah was getting at, thus answering the unstated questions on the minds of the priests.
"I baptize with water;
but there is one among you whom you do not recognize,
the one who is coming after me,
whose sandal strap I am not worthy to untie."
For further details on how John prepares the way of the Lord, see Matthew 3 and Luke 3. Hint: John is preaching repentance, and symbolizing that repentance with a baptism with water. He also gives specific advice to people how to make that repentance concrete. And he speaks of what the Lord will bring.
Only God can see the future. Indeed, he who created space and time, and all that is in it, sees past, present and future. He gives his prophets visions of the future to guide us on the way of salvation. He established successive covenants in the Old Testament (e.g., the Covenant with Noah and his progeny, the Covenant with Abraham, Isaac and Jacob, and the 12 tribes issuing from them, the Covenant with Moses and Joshua, and the 12 tribes freed from slavery in Egypt and brought to the Promised Land, the Covenant with David and Solomon and their successors), all of which were preliminary, signs pointing to the New and Final Covenant.
To understand and appreciate the meaning and scope of the New Covenant, it helps to decode the symbolism of these Old Testament covenants. We can begin by asking meaning and purpose questions. We can ask "Who?", "Why?" and other such questions.
Better than that, we can get answers. In effect, the Old Testament inspires questions. The New Testament gives answers to those questions. To put it in the traditional way, "The New Testament lies hidden in the Old and the Old Testament is unveiled in the New." [This is a quotation from St. Augustine, cited in the Catechism of the Catholic Church, paragraph 129. See http://ccc.usccb.org/flipbooks/catechism/index.html#36.]