As members of the Church of Jesus Christ of Latter-day Saints, we know that a prophecy from the Old Testament about Elijah (Elias) returning before Christ returns to the Earth has been fulfilled. The prophecy is from Malachi 4:
5 ¶Behold, I will a you b the prophet c the coming of the d and dreadful e of the Lord:
6 And he shall a the b of the c to the d, and the heart of the e to their fathers, lest I come and fthe g with a h.
Often other churches use a statement by Jesus about John in which Jesus says John was an Elias (one who comes before). This usually happens because the common church member doesn't realize that Elias can be used as a title and that the specific name Elijah in Malachi does refer to the prophet Elijah. Also, if they believe John the Baptist is Elijah, they essentially believe the same person can be born twice, which is something I have never heard a Christian religion teach.
The other church members also don't often know about or remember this verse from John 1 about John the Baptist:
21 And they asked him, What then? Art thou a? And he saith, I am not. Art thou that prophet? And he answered, No.
22 Then said they unto him, Who art thou? that we may give an answer to them that sent us. What sayest thou of thyself?
23 He said, I am the a of one crying in the wilderness, Makeb the c of the Lord, as said the prophet Esaias.
John the Baptist was fulfilling a different prophecy from Isaiah, not the one from Malachi. If John the Baptist was Elijah the prophet, he either didn't know it or he lied about it. I don't think either is likely. Instead, John was acting like Elijah in preparing the way of the Lord but in this way was only a type (or symbol of) Elijah coming in the last days. Here's a verse explaining he would act "in the spirit of Elias" from Luke 1:
17 And he shall go before him in the spirit and power of a, to turn the hearts of the b to the children, and the cto the wisdom of the just; to make ready a people d for the Lord.
Finally, this is when Elijah came back to the Earth, as recorded in the D.C. 110:
13 After this vision had closed, another great and glorious aburst upon us; for b the prophet, who was taken to heaven without tasting death, stood before us, and said:
14 Behold, the time has fully come, which was spoken of by the mouth of Malachi—testifying that he [Elijah] should be sent, before the great and dreadful day of the Lord come—
15 To a the b of the fathers to the children, and the children to the fathers, lest the whole earth be smitten with a curse—
16 Therefore, the a of this b are committed into your hands; and by this ye may know that the great and dreadfulc of the Lord is near, even at the doors.
The Earth is being prepared for the Second Coming of the Lord. Elijah has returned. I love seeing the church of Jesus Christ of Latter-day Saints progress so that more people can be prepared for his coming.
These verses from Matthew 17 causes confusion, but taken in context with other verses from the Bible and Latter-day revelation, it is clear who Elias and John the Baptist are/were:
12 But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they a. Likewise shall also the Son of man suffer of them.
13 Then the disciples understood that he spake unto them of John the Baptist.
Have a great day.