From idontknownuthin

Mt 11:14

And if ye will receive it, this is Elias, which was for to come.

Matt 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.

13 Then the disciples understood that he spake unto them of John the Baptist.

How could Jesus say that John was Elijah?

The shadows must show how John is Elijah and how "This is Elijah".

The answer has three parts. John is Elijah, John and Jesus are Elijah and Elisha, and Elijah and Elisha are Christ. The hint that this is so is that Elijah and Elisha shared the mantle and their ministries overlapped a short time. John and Jesus both preached repentance (turned the hearts of the children to the fathers) and their ministries overlapped. Tons of details will be exposed as we continue.