It’s not what John said about the priests that would land him in hot water, it’s what he said about himself. You’ll notice the same thing about the person John was preparing the way for. The priests didn’t start picking up stones when Jesus insulted them. It was when Jesus made claims about himself that He fulfilled prophecy.

John could call the priests a generation of vipers or any number of other things to their faces without serious consequence. He probably wasn’t alone. It’s probably a bit like calling a politician sleazy. The priests were probably used to it.

But if John says of himself that He is the fulfillment of the prophecy of the return of Elijah, that becomes much more serious. Now the priests are thinking “Is this man sane or is he a blasphemer?”

John says he is the figure in Malachi 3:1, but at the same time denies that he is the figure in Malachi 4:5—Elijah. This sends a mixed signal to his questioners. They, relying on their scriptures, are expecting Elijah to precede the Messiah. So when John tells them he isn’t Elijah, he is at the same time telling them that the one coming after him isn’t the Messiah. Instead of gathering with Jesus, John is scattering. This clearly
demonstrates that John is not a prophet sent by a god who is “not the author of confusion”.

John may have lied for a number of reasons. The most obvious being fear of the questioners. Scripture truthfully reports this lie. It also records that when he is pressed for a direct description of who he is, he ends up identifying himself as the fulfillment of Malachi’s prophecy after all. But he does so in a roundabout way that avoids his directly saying of himself what Jesus says of him.

When asked directly who he is, John says that he is the voice calling mentioned in Isaiah 40:3. This does two things:

1. It identifies Jesus as the one God of Israel. (Isaiah says that it is the LORD whose way is to be prepared, but John was clearly preparing the way for Jesus.)

2. It identifies John as the forerunner, prophesied in Malachi 3:1, who would go before the coming of the one God to prepare the way. Malachi calls this forerunner Elijah (Isaiah describes the voice calling as one who is preparing the way for the LORD).

Item 2 gives the lie to John’s denial.