Matthew 17:13: Then the disciples understood that he was speaking to them of John the Baptist.
The second Elijah (John the Baptist [Matthew 11:1-19, 17:9-13]) was to come before the Messiah (Jesus) according to the scriptural analysis of the religious leaders in Israel. They were accurate in their interpretation but that was not enough to recognise who those divine players were, missing it on both counts. They had read the scriptures but do not know the God of the scriptures, so they could not recognise his working on the earth.
There is a difference between walking by revelation and walking by regulation. The latter is when you think the scriptures is an end in itself and not a means to an end in a dynamic, continually unravelling of the person of Jesus.
There is a difference between the Pharisee who can quote the scriptures well and Simeon the old man who by revelation was told by the Holy Spirit that he will not die until He saw Jesus (Luke 2:25-35). The latter recognised Jesus even as a baby the former nailed him on the cross as a grown man, in an eternal testament to their blindness.
When an angel was speaking about the birth of said he would before the Lord (Messiah) in the spirit and power of Elijah (Luke 1:13-17, Matthew 3:11-12). The first Elijah called down fire (1Kings 17-19). However if you were looking for the second Elijah that Jesus showed the people was John the Baptist as prophesied by Malachi (Malachi 4:5), you would get it wrong if they were looking for someone who calls down fire.
Elijah manifested the first time calling the people to repentance through withholding water but John the Baptist came using water to baptise the people unto repentance and he will be the first one to speak of the baptism of the Holy Spirit (Luke 3:16), which will birth people into devotion to God. Elijah came with physical fire, John the Baptist the second Elijah spoke about a spiritual fire of purification which Jesus is going to bring, through baptism us in the Holy Spirit, in an eternally lasting experience of God.
Fire is definitely is a symbol of the Holy Spirit as that is the form in which he appeared in the upper room in Jerusalem and alighted on the disciples and they were baptised, immersed in him, taken over by him (Acts 2:1-4).
That is why the initial evidence of the baptism of the Holy Spirit is speaking in tongues, because whatever takes control of your tongue has taken control of your life (James 3:2). The bible says that cloven tongues as of fire appeared and rested on everyone and they began to speak in new tongues as the Spirit gave them utterance.
The Pharisees could not know John’s identity as the promised Elijah if they were looking for fire. And since they believed that Elijah was going to come before the Messiah manifest, not being able to recognise the Elijah meant they would not recognise Jesus to fatal consequences.
They could not discover who Elijah was by observing John the Baptist, nor could they achieve it by asking him if he was the promised Elijah, wondering if he fitted into their religious construct. His answer: no I am not (John 1:19-36).
He was not lying. Though he was Elijah he was not told that was who he was. Not until he was in prison and after his death did Jesus revealed that to the people come to such a conclusion.
John the Baptist moved in the spirit and the power of Elijah. While Elijah faced the threat of death from the hand of Jezebel, John the Baptist would eventually be beheaded through the demand of a woman, who was married to the king ruling Israel (just like Jezebel was married to King Ahab).
Elijah wore a cloth made of animal skin, which was what John the Baptist put on (Matthew 3:4). Both had strong prayer lives.
We read that John the Baptist taught his disciples to pray (Luke 11:1) and that they regularly fast (Mark 2:18). Elijah was also iconic when it comes to prayer. James refers to him as an ordinary human being with an extraordinary prayer life which made all the difference, able to seize rain for years and return the same through prayer (James 5:16-18).
Jesus said about John that he was a prophet and more than a prophet, and that of those born of a woman none is greater than John the Baptist, but the least in God’s kingdom is greater than him.
So no matter how great the friend of the bridegroom (the way John the Baptist referred to himself in relation to Jesus) is (John 3:22-30), he cannot be greater than the bride (the church), who is meant to be one with the bridegroom (Jesus). The friend has a limited role while the bride has an eternal role.
Jesus is in us and that makes all the difference. John spoke of Jesus, revealed Jesus but we have him. We are the one who gets to experience baptism in fire through Jesus. We get to experience Jesus in his fullness, in spiritual life within us.
John spoke about him we experience him. He was related to him naturally, we are related to him spiritually. He knew him as Lamb of God, we know him as God.
At a time, he expressed doubts about Jesus. While in prison he sent people to Jesus to ask: are you the one to come or shall we look for another?
He doubted his own statements, his earlier revelations about Jesus. In his assumption he had viewed things through the prevailing prophetic glasses of his time, which states that the Christ will come and rid Israel of the oppression of the Roman (and any other), ushering in a time of bliss for Israel (Jesus came for something bigger, peace between creator and people from all nations). But it was not to be at the time he expected it, how he expected it and he was disillusioned, as he was manhandled and committed to prison by the oppressive, godless Roman regime that he expected Jesus to overthrow.
John got disillusioned when things were not going according to his assumption, not knowing that God is greater than his assumption.