I was just curious why it is that when John the Baptist identifies himself in the Gospel of John as the one who was a voice crying out in the desert he attributed the scripture to Isaiah and not Malachi?
It’s true that if John had used the quote from Malachi 4:5 it would have left no doubt as to his identity. But it would have also signaled the beginning of the Great Tribulation, which is the theme of Malachi 4.
But from the way both he and Jesus spoke about his identity, it appears that John would have fulfilled the prophecy of Elijah only if the people were ready to accept Jesus as their Messiah. In Matt. 11:14 Jesus said, “And if you are willing to accept it, he (John) is the Elijah who was to come.”
John, knowing that they weren’t ready, denied being Elijah in John 1:21 even though the Angel Gabriel had said that he would minister in the spirit and power of Elijah (Luke 1:17). That left him the less specific quote from Isaiah 40:3 as one who came to herald the coming Messiah without completely fulfilling the role of Elijah.
It should be noted that the final verse of the Old Testament, Malachi 4:6 says that if the people refused Elijah, God would strike the land with a curse. In that sense, it may be that John could have used the Malachi prophecy after all.