A.T. Robertson Commentary Mark 9:12

"And he said unto them, Elijah indeed cometh first, and restoreth all things: and how is it written of the Son of man, that he should suffer many things and be set at nought?" — Mark 9:12 (ASV)

Restoreth all things (αποκατιστανε παντα). This late double compound verb, usual form αποκαθιστημ in the papyri, is Christ's description of the Baptist as the promised Elijah and Forerunner of the Messiah. See on Mt 17:10-13. The disciples had not till now understood that the Baptist fulfilled the prophecy in Mal 3:5f. They had just seen Elijah on the mountain, but Jesus as Messiah preceded this coming of Elijah. But Jesus patiently enlightens his dull pupils as they argue about the exegesis of the scribes.