NOTES for Mat 17:10-13
By asking about Elijah, the disciples are trying to find out from Jesus whether His coming corresponds to the messianic prophecies. If He is the Messiah, He should have been preceded by the second coming of the great prophet. Christ's answer makes them shift their perspective somewhat. The Lord does not say that Elijah has already come and therefore that He is the awaited Savior. He says that John the Baptist was that very forerunner, but directs attention not to the fact of His own messiahship that follows from this, but to how people respond to this coming. The death and humiliation endured by John await Him as well. In preparing the apostles for this, He draws their attention to the Messiah's sacrificial role. Perhaps many of them had already become disappointed in the Forerunner after his execution, regarding him as an ordinary man. These words of Christ help them look at the Baptist anew and see that he was chosen by God, though not in the usual worldly sense.