Mt 17: 9a. 10-13
As they were coming down from the mountain, Jesus charged them, “Do not tell the vision to anyone until the Son of Man has been raised from the dead.”
Then the disciples asked him, “Why do the scribes say that Elijah must come first?”
He said in reply, “Elijah will indeed come and restore all things;
but I tell you that Elijah has already come, and they did not recognize him but did to him whatever they pleased. So also will the Son of Man suffer at their hands.”
Then the disciples understood that he was speaking to them of John the Baptist.
When they came to the crowd a man approached, knelt down before him,
and said, “Lord, have pity on my son, for he is a lunatic and suffers severely; often he falls into fire, and often into water.
I brought him to your disciples, but they could not cure him.”
Jesus said in reply, “O faithless and perverse generation, how long will I be with you? How long will I endure you? Bring him here to me.”
Jesus rebuked him and the demon came out of him, and from that hour the boy was cured.
Source: New American Bible
Catechism of the Catholic Church
718 John is “Elijah [who] must come.” The fire of the Spirit dwells in him and makes him the forerunner of the coming Lord. In John, the precursor, the Holy Spirit completes the work of “[making] ready a people prepared for the Lord.”