Who is this "John" who was sent from God? John, first of all, was prophesied to come in the Tanakh:

"Behold, I am going to send My messenger, and he will clear the way before Me. And the Lord, whom you seek, will suddenly come to His temple; and the messenger of the covenant, in whom you delight, behold, He is coming," says the LORD of hosts. Malachi 3:1 NASB

Who is Malachi referring to as the messenger? The word "messenger" is malak, which is often translated as: "angel." But here it refers to Elijah:

"Behold, I am going to send you Elijah the prophet before the coming of the great and terrible day of the LORD. Malachi 4:5 NASB

So the prophecy in Malachi speaks of Elijah, but there is nothing more distinctly affirmed in the New Testament than the identity of John the Baptist as the Elijah of Malachi:

And His disciples asked Him, saying, "Why then do the scribes say that Elijah must come first?" 11 And He answered and said, "Elijah is coming and will restore all things; 12 but I say to you, that Elijah already came, and they did not recognize him, but did to him whatever they wished. So also the Son of Man is going to suffer at their hands." 13 Then the disciples understood that He had spoken to them about John the Baptist. Matthew 17:10-13 NASB

View sermon transcript or here audio version here: http://www.bereanbiblechurch.org/transcripts/john/john_01_06-08_john-the-witness_video.php

02/21/16 - Pastor David B. Curtis - Berean Bible Church - http://www.bereanbiblechurch.org - Message #797