9 As they were coming down from the mountain, 10 his disciples asked him, saying, “Then why do the scribes say that Elijah must come first?”

11 Jesus answered them, “Elijah indeed comes first, and will restore all things, 12 but I tell you that Elijah has come already, and they didn’t recognize him, but did to him whatever they wanted to. Even so the Son of Man will also suffer by them.” 13 Then the disciples understood that he spoke to them of John the Baptizer.

Matthew 17:9a, 10-13 - World English Bible

In Matthew's gospel, Jesus and a few of his disciples (Peter, James and John) came down the mountain after the Transfiguration (Matthew 17:1-9). The missing verse from the passage above was: Jesus commanded them, saying, “Don’t tell anyone what you saw, until the Son of Man has risen from the dead.” (Matthew 17:9b). The two events (Transfiguration and the prediction of his Passion) reinforced their belief about the Master: he was the Christ. Now they asked the question about Elijah.

In 1 Kings 2:8, Elijah did not suffer death but was whisked into heaven on a chariot of fire; in the time of Jesus, people expected the prophet to return and reveal the Messiah (see Malachi 4:5-6). So, the disciples expected some Elijah figure. The question was: who was he? Jesus implicitly answered, "John the Baptist."

The core of the passage lay in 17:12, where Jesus declared that both he and John would not be recognized by the leadership and both would be ill-treated. What happened to the Baptizer would happen to the Messiah.

When have you failed to see Christ in others? How can you rectify that error?