Elijah has come already and they did not recognise him
As they came down from the mountain the disciples put this question to Jesus, ‘Why do the scribes say that Elijah has to come first?’ ‘True;’ he replied ‘Elijah is to come to see that everything is once more as it should be; however, I tell you that Elijah has come already and they did not recognise him but treated him as they pleased; and the Son of Man will suffer similarly at their hands.’ The disciples understood then that he had been speaking of John the Baptist.
The prophet Elijah will come again
The prophet Elijah arose like a fire,
his word flaring like a torch.
It was he who brought famine on the people,
and who decimated them in his zeal.
By the word of the Lord, he shut up the heavens,
he also, three times, brought down fire.
How glorious you were in your miracles, Elijah!
Has anyone reason to boast as you have?
Taken up in the whirlwind of fire,
in a chariot with fiery horses;
designated in the prophecies of doom
to allay God’s wrath before the fury breaks,
to turn the hearts of fathers towards their children,
and to restore the tribes of Jacob,
Happy shall they be who see you,
and those who have fallen asleep in love.
Source: Jerusalem Bible
Catechism of the Catholic Church
John, precursor, prophet, and baptist
717 “There was a man sent from God, whose name was John.” John was “filled with the Holy Spirit even from his mother’s womb” by Christ himself, whom the Virgin Mary had just conceived by the Holy Spirit. Mary’s visitation to Elizabeth thus became a visit from God to his people.
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.”
719 John the Baptist is “more than a prophet.” In him, the Holy Spirit concludes his speaking through the prophets. John completes the cycle of prophets begun by Elijah. He proclaims the imminence of the consolation of Israel; he is the “voice” of the Consoler who is coming. As the Spirit of truth will also do, John “came to bear witness to the light.” In John’s sight, the Spirit thus brings to completion the careful search of the prophets and fulfills the longing of the angels. “He on whom you see the Spirit descend and remain, this is he who baptizes with the Holy Spirit. And I have seen and have borne witness that this is the Son of God. . . . Behold, the Lamb of God.”
720 Finally, with John the Baptist, the Holy Spirit begins the restoration to man of “the divine likeness,” prefiguring what he would achieve with and in Christ. John’s baptism was for repentance; baptism in water and the Spirit will be a new birth.
A psalm of Asaph. O God, the nations have invaded your heritage; they have defiled your holy temple, have laid Jerusalem in ruins.
They have left the corpses of your servants as food for the birds of the heavens, the flesh of your faithful for the beasts of the earth.
They have spilled their blood like water all around Jerusalem, and no one is left to bury them.
We have become the reproach of our neighbors, the scorn and derision of those around us.
How long, LORD? Will you be angry forever? Will your rage keep burning like fire?
Pour out your wrath on nations that reject you, on kingdoms that do not call on your name,
For they have devoured Jacob, laid waste his home.
Do not hold past iniquities against us; may your compassion come quickly, for we have been brought very low.
Help us, God our savior, for the glory of your name. Deliver us, pardon our sins for your name’s sake.
Why should the nations say, “Where is their God?” Before our eyes make clear to the nations that you avenge the blood of your servants.
Let the groans of prisoners come before you; by your great power free those doomed to death.
Lord, inflict on our neighbors seven fold the disgrace they inflicted on you.
Then we, your people, the sheep of your pasture, will give thanks to you forever; through all ages we will declare your praise.
Source: The New American Bible