Matthew 17:10

His disciples - instead of His disciples, some MSS., with the Coptic, Armenian, Vulgate, all the Itala except two, and Origen, read simply, οι μαθηται, The disciples, i.e. those only who had been with him on the mount, Peter, James, and John.

Why then say the scribes that Elias must first come? - As the disciples saw that Elijah returned to heaven, knowing the tradition of the elders, and the prophecy on which the tradition was founded, Malachi 4:5, Malachi 4:6, Behold I send you Elijah the prophet, before the great and terrible day of the Lord shall come; and he shall turn the hearts, etc., it was natural enough for them to inquire what the meaning of the tradition, and the intention of the prophecy, were.

See also Mark 9:11-13.

Why then say the scribes ... - The disciples appear to have been satisfied now that he was the Messiah. The transfiguration had taken away all their doubts, but they recollected that it was a common doctrine among the Jews that Elijah would appear before the Messiah came, and they did not then recollect that he had appeared. To this difficulty the word then refers. "We are satisfied that thou art the Christ, but Elijah has not yet come, as was expected; what, then, is the meaning of the common opinions of our learned men, the scribes? Were they right or wrong in their expectation of Elijah?" See the notes at Matthew 11:14.

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?