19 Grk “he”; the referent (John) has been specified in the translation for clarity.
20 The crowds. It is interesting to trace references to “the crowd” in Luke. It is sometimes noted favorably, other times less so. The singular appears 25 times in Luke while the plural occurs 16 times. Matt 3:7 singles out the Sadducees and Pharisees here.
21 Or “snakes.”
22 The rebuke “Who warned you to flee …?” compares the crowd to snakes who flee their desert holes when the heat of a fire drives them out.

In other versions

He said therefore to the crowds that came out to be baptized by him, “You brood of vipers! Who warned you to flee from the wrath to come?

John said to the crowds coming out to be baptized by him, “You brood of vipers! Who warned you to flee from the coming wrath?

He then said to the crowds who came out to be baptized by him, “Brood of vipers! Who warned you to flee from the coming wrath?

When the crowds came to John for baptism, he said, “You brood of snakes! Who warned you to flee the coming wrath?

Therefore he was saying to the crowds that came out to be baptized by him, “Offspring of vipers! Who warned you to flee from the coming wrath?

Then said he to the multitude that came forth to be baptized of him, O generation of vipers, who hath warned you to flee from the wrath to come?

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