Matthew 17, and what Jesus said about Elijah:
The scriptures tell us that Jesus drank from the cup that was passed around the Passover table, and his blessing became what Christians call the Eucharist.
And, that evening, from the cross Jesus, cried out to Elijah according to the witnesses at the Cross.
Just a few evening before that event, Elijah is seen by two apostles leaving Jesus and joining back to Moses.
This is a ritual of metamorphosis, (according to the definition of the Greek word the English Bible calls a "transfiguration").
The Christ is gone, returned to Moses who he was very much akin as a miracle worker, [Deuteronony 18:15-20].
And Jesus returns, the son-of-man, is back in the flesh, ready to die on the Cross.
Coming down from the mountain Jesus tells his apostles that Elijah had come, was mistreated and ignored. bJesus means that Elijah had just left to return to Moses.
But the apostles who did NOT see the transfigurating metamorphosis of the evening just past do not understand.
They assume he refers to John who had been beheaded.