“And his disciples asked him, saying, Why then say the scribes that Elias must first come?”
King James Version (KJV)
17:10 Why then say the scribes, that Elijah must come first - Before the Messiah? If no man is to know of his coming? Should we not rather tell every man, that he is come, and that we have seen him, witnessing to thee as the Messiah?
Mt 17:10 Why then say the scribes that Elijah must first come? They knew that the scribes, in their capacity as interpreters of prophecy, were wont to say that Elijah must come before the Messiah could appear. They said this on the strength of Mal 3:1 4:5. If Peter, James and John were of those who asked this question, they were probably seeking to ascertain if the vision they had seen was the coming of Elijah and why he did not remain.