Matthew 17:13

Then the disciples understood that He spake unto them of John the Baptist.

The fact that they had seen the prophet Elijah in the vision on the mountain recalled to their minds the saying of the scribes, probably based on Malachi 4:5, as to the coming of Elijah. Their understanding was that Elijah would reappear in person, settle the quarrels between the various Jewish schools, bring back the pot of manna and Aaron's rod, and sanctify the people by an extraordinary washing. Jesus concedes the correctness of the idea: Elijah, according to the prophecy, was indeed to come for the purpose of restoring everything among the Jews to its proper state, as the Lord wanted it to be. He was to prepare the way for the Lord Himself. But the Lord finds fault with the fact that the scribes and the Jewish people in general did not recognize the second Elijah as such, but did what they pleased with him. The leaders of the people rejected him, and the dissolute, adulterous tetrarch put him to death. He shared the fate of most prophets that place the fearless confession of truth above the concern for their own safety and welfare. From the rejection of His herald to the denial of the Messiah Himself is only a small step; and even in the same manner will they cause Him to suffer. This explanation was sufficient to open the eyes of the disciples; they understood that John the Baptist was the Elijah who was to come before the great and dreadful day of the Lord.