John wanted the crowds to stop and ask why they had really come. They were coming to be baptized, but a ritual without a changed life would not save anyone from "the wrath to come." Calling them a "generation of vipers" was a sharp rebuke; Barnes, on the same words in Matthew, says the viper can be "the symbol of wickedness, of envenomed malice," and thinks that is probably the sense there. His question is rhetorical. He was not asking for a name. Barnes hears in it John's astonishment that such sinners "should have been induced to flee from coming wrath." His next words show the point: "Bring forth therefore fruits worthy of repentance" (Luke 3:8).

John answered his own question with a command: "Bring forth therefore fruits worthy of repentance" (Luke 3:8). He warned them not to trust their ancestry: "begin not to say within yourselves, We have Abraham to our father," for "God is able of these stones to raise up children unto Abraham" (Luke 3:8). The danger was near: "now also the axe is laid unto the root of the trees" (Luke 3:9). The crowd took it to heart and asked, "What shall we do then?" (Luke 3:10).

Luke dates John's ministry carefully: "the fifteenth year of the reign of Tiberius Caesar," when Pontius Pilate governed Judaea and "Annas and Caiaphas" were high priests (Luke 3:1-2). John's work fulfilled Isaiah's prophecy of "The voice of one crying in the wilderness, Prepare ye the way of the Lord" (Luke 3:4; Isaiah 40:3). Matthew records the same question but says John spoke it when he saw "many of the Pharisees and Sadducees" coming to his baptism (Matthew 3:7). Luke addresses it to "the multitude" as a whole (Luke 3:7). Gill reads the two accounts together and says the multitude addressed here "were many of the Pharisees and Sadducees." The KJV "generation of vipers" is "brood of vipers" in the ESV: the offspring of poisonous snakes.

This question shows that God takes sin and judgment seriously, and that He warns before He judges. "The wrath to come" is real, and John's message was urgent: the axe was already at the root (Luke 3:9). But the warning itself is mercy. God sent John so that people could flee and be ready. Matthew Henry writes, "There is no way of fleeing from the wrath to come, but by repentance."

John also exposed a false security: "We have Abraham to our father" (Luke 3:8). Belonging to God's people by birth did not replace a turned heart. Instead of ancestry, John pointed to the One coming after him, who would "baptize you with the Holy Ghost and with fire" (Luke 3:16).