Then the disciples understood that he spake unto them of John the Baptist. Then the disciples understood - When he spoke of the sufferings of this prophetic Elijah, and also of his own, which had been the subject of the conversation on the mount, during the transfiguration, they clearly apprehended that he spoke of John the Baptist.
Related Commentaries of Matthew 17:13
Matthew 17:13
13 Then the disciples understood that he spake unto them of John the Baptist.