Indeed, Elijah is coming first and will restore all things. But I say to you that Elijah has come already, and they did not know him but did to him whatever they wished. Likewise the Son of Man is also about to suffer at their hands. Then the disciples understood that He spoke to them of John the Baptist.
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The interesting part of that is 'elijah is coming first'. I don't know what he means cuz the next sentence contradicts it. I dunno.. I remember reading that quote before and being puzzled by it...
Perhaps he speaks of Elijah coming first again in his second coming. Perhaps Elijah is one of the two witnesses in Revelations. Some think so. They think the other is Moses, and reference to Jude where Lucifer and Michael fight over the body of Moses. Also the miracles that are performed by the two witnesses were only done by these two prophets, to turn water to blood(Moses) and to stop the rains(Elijah).