Matthew 17:10-13 ESV
And the disciples asked him, “Then why do the scribes say that first Elijah must come?” He answered, “Elijah does come, and he will restore all things. But I tell you that Elijah has already come, and they did not recognize him, but did to him whatever they pleased. So also the Son of Man will certainly suffer at their hands.” Then the disciples understood that he was speaking to them of John the Baptist.
Jesus tells the disciples that Elijah has come (present tense) and he will restore (future tense) all things.
Elijah and Moses foreshadow the coming Christ. At this point of time the fullness of God is manifest in the body of Jesus. Elijah and Moses disappear leaving Jesus, the Son of God and the light of the world. They will appear again to prepare the way for the coming Christ.
They will return to give witness to Israel and the world that the Lord is God, that they would know. They will refocus the attention back to Israel after the coming and going of the church and will signal the final days of the wrath of God.
The future lies in the past and the past speaks of the future. It's in the word, the beginning and the end.