Matthew 17:12
But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them. But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them.
Elijah has already come in the person and ministry of John the Baptist. He was not recognized and accepted, but those who should have received him did to him as they pleased, including the people and the religious leaders, priests and rulers. In the same way the Son of man would suffer at their hands, being treated as they pleased in rejection and condemnation. His rejection and condemnation were foretold by Scripture, yet Israel would still be given another opportunity to accept the Son of God.
The disciples believed Elijah had come as prophesied. They wrongly took the transfiguration vision to mean Christ's kingdom would now be set up on earth and heavenly messengers would shield Jesus from what awaited him in Jerusalem. They forgot that the plan of salvation could be completed only through his suffering and death. His suffering and crucifixion were the great final sacrifice toward which the temple offerings and services had pointed, so that when the sacrifice was completed the types met their antitype. Animal sacrifices and their ceremonies no longer had value before God because he had offered himself to redeem the world from sin, and the tearing of the temple veil at his death signified that the ceremonial sacrificial system had ended.
Standing with Christ on the mount means beholding his beauty and glory until the soul becomes radiant with his glory. It means then going down to the plain where souls enslaved by Satan wait for the word of faith and prayer to set them free. It means drinking most deeply of his self-sacrificing love that lives and labors and sacrifices even unto death for saving humanity.