Gospel according to Saint Matthew 17, 10-13
As they were coming down the mountainside, the disciples then asked Jesus, «Why do the teachers of the Law say that Elijah must come first?». And Jesus answered, «So it is: first comes Elijah to set everything as it has to be. But I tell you, Elijah has already come and they did not recognize him, but treated him as they pleased. And they will also make the Son of Man suffer». Then the disciples understood that Jesus was referring to John the Baptist.
