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.
17:9 Jesus’ coming down from the mountain with his disciples continues the Sinai parallels, recalling Moses who came down the mountain (Exod 34:29).
17:10–13 In popular belief and scribal teaching Elijah was expected to return in some sense in the last days and prepare the way for the Lord (Mal 3:23–24; Sir 48:10). Since Peter, James, and John just saw Elijah speaking with Jesus, they wonder if that expectation has now been fulfilled. Jesus agrees with the popular belief, saying Elijah will indeed come and restore all things, a reference to Elijah’s ministry of preparing the people for the Lord by calling them to repentance and mending kinship relationships wounded by sin (Mal 3:23–24). However, Jesus adds the contrasting phrase, but I tell you. As in 5:22, these words denote a contrast in which Christ offers something in addition to what was taught previously. While Jesus endorses the scribal expectation of Elijah’s return, he adds that Elijah has already come. In other words, the scribes and many others in Israel were right to look for Elijah’s return but, unfortunately, they have missed his coming. The great eschatological reappearance of Elijah has already taken place in the person and ministry of John the Baptist (11:14; 17:13; see 3:4). Like Elijah, John was a great prophet who called the people to repentance and was persecuted by a wicked king (see 14:3–12).
Jesus uses this discussion about the suffering of the new Elijah, that is, John the Baptist, to return to the theme of his own suffering (see 16:21). Just as the new Elijah was opposed by many in Israel, so will the Son of Man suffer at their hands.
Today’s Gospel passage identifies the appearance of John the Baptist with the expected return of the prophet Elijah. John, the herald of Christ, appears in the desert. Here he stands for all of us in the desert of sin, the lifeless place. It is as though John purposely went there to remind us of our need for grace.
What is he proclaiming? A baptism of repentance. This is the great message. Turn your life over to a higher power. People are coming to him from all sides, because, in our heart of hearts we all resonate with this message.
So often in the Old Testament, the prophets are asked to act out some quality of the people, perhaps something they were unable or unwilling to see. Well, this tradition continues here: John acts out for the people their helplessness and neediness before the Lord. But then like Isaiah, he refuses to leave it at that. He announces that one is coming, one who will baptize in the Holy Spirit.
– Bishop Robert Barron
May the virtues of faith, hope, and love go with you today – DV.
 Curtis Mitch and Edward Sri, The Gospel of Matthew, Catholic Commentary on Sacred Scripture (Grand Rapids, MI: Baker Academic, 2010), 219–220.