Theophylact of Ohrid — as excerpted in the Catena Aurea on Mark 9:9-13

“Now the Lord puts this forward to oppose the notion of the Pharisees, who held that Elias was the forerunner of the first advent, shewing that it led them to a false conclusion; wherefore he subjoins, And how it is written of the Son of man, that he must suffer many things, and be set at nought. As if He had said, When Elias the Tishbite cometh, he will pacify the Jews, and will bring them to the faith, and thus be the forerunner of the second advent. If then Elias is the forerunner of the first advent, how is it written that the Son of man must suffer? One of these two things therefore will follow; either that Elias is not the forerunner of the first advent, and thus the Scripture will be true; or that he is the forerunner of the first advent, and then the Scriptures will not be true, which say that Christ must suffer; for Elias must restore all things, in which case there will not be an unbelieving Jew, but all, whosoever hear him, must believe on his preaching.”