As they were coming down from the mountain, the disciples asked Jesus, “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.
Guercino. 1661. St. John the Baptist at the Well.